INTRODUCTION This Quality Metal Finishing Guide deals with pretreatment of surfaces for metallic coatings. In this context, pretreatment is defined as the process of substrate preparation to receive a specific metallic coating. Generally, the objective of applying a metallic coating is to provide one or several new properties to a basis metal. Coatings include decorative finishes such as copper or brass on zinc diecast, and functional coatings such as hard chrome or electroless nickel on aluminum, or corrosion resistant zinc alloys on steel. Regardless of the function, these coatings all must adhere completely to the basis metal if they are to provide the decorative or functional properties required. Plate adhesion must exceed the cohesive strength of either the substrate or the electroplate. In other words, an attempt to peel the metallic coating from the substrate should remove a layer of the basis metal without separation of the coating from the substrate at the boundary. In plating, this level of adhesion is possible between the atoms of the substrate and the plated metal. Whatever the mechanism, adhesion depends on the condition of the surface being plated. Pretreatment, therefore, consists of the critical steps used in the removal of any undesirable contaminants that might interfere with the inter-atomic adhesion. Typical contaminants are soils such as oils, grease, fingerprints, oxides and scale. Pretreatment also affects other coating properties. Even if adhesion is secured initially, blistering, spotting out, stress in deposit, and failure due to hydrogen embrittlement occurring at a later date may be related to surface preparation. Premature failure resulting from corrosion can be avoided only by perfect surface preparation prior to complete coverage by the specified coating. Automotive and other industries rely on the integrity of plated components in providing long-term product-performance warranties. The primary selection of a given pretreatment process is dictated by two factors. 1. THE METAL BEING PROCESSED AND THE CLEANING SOLUTION Generally, steel and copper can withstand strongly alkaline cleaners (pH 10-14), and acid pickles such as 50% hydrochloric hydrochloric acid, with little or no attack of the basis metal. Bronze and brass require mild cleaners (pH 9-13) and lower lower temperatures to prevent the selective removal of the zinc from the alloy (dezincification). Zinc and aluminum require yet milder inhibited alkalis (pH 912) and mild acids. Proprietary formulations are available to meet these needs. 2. THE TYPE OF SOILS TO BE REMOVED Soil is any substance on the surface of the metal that will interfere with the metal-finishing process being performed. Soil may be the result of intentional application to the metal of a material that facilitates metal working, handling, and exposure to high temperature or the atmosphere. Generally speaking, the types of soils can be broken down into two broad categories: organic and inorganic. a. Organic Saponifiable animal and vegetable oils. Unsaponifiable mineral oils and waxes. Miscellaneous contaminants either formed in situ (metallic soaps formed in the buffing operation), or inhibitors from certain acid pickling solutions redepositing on the metal. b. Inorganic Scale and smut, oxide and metallic residues. Polishing compounds, abrasive, grinding and polishing residues or grits. Miscellaneous shop dusts and soldering fluxes. These are only a few of the soils encountered. Some of the others include paints, cleaning residues such as water-in-oil emulsions, fingerprints, and inorganic coatings such as phosphates, chromates and rust preventatives.
Many of the lubricants, coolants and corrosion preventives used today are soluble or emulsifiable. These soils are best cleaned in a mild, non-caustic-soda-containing alkaline cleaner at low temperatures. In fact, many water-soluble lubricants can be removed partially by a warm water pre-rinse, thereby reducing cleaner contamination and minimizing cleaning problems. Particulate matter is also more readily removed by mild, low-temperature cleaning. MECHANISMS OF SOIL REMOVAL Most cleaning and soil removal is accomplished by one or more of the following mechanisms: 1. WETTING The process by which the cleaner, through the use of surface-active agents, loosens the metal-tosoil bond by the displacement of oil and the lowering of surface and interfacial tensions. Wetting, in essence, is the first requirement for cleaning soiled metal. 2. EMULSIFICATION Once wetting takes place, the process of emulsification can occur. This is the dispersion of two mutually immiscible liquids. Emulsification is dependent on the oil encountered and the choice of surfactants, and secondarily on factors such as pH, temperature and agitation. 3. SAPONIFICATION The reaction of fatty acids with hot alkalis to produce soluble or emulsifiable soaps. Examples are the saponification of buffing compounds and stearate drawing compounds. 4. SOLUBILIZATION The process by which the solubility of a substance (oil) is increased in a certain medium, e.g., water, by the proper selection of surface active agents. Many oils used today are, in fact, water soluble. 5. DEFLOCCULATION The process whereby the soil is broken into very fine particles and dispersed in the cleaning media. This soil is then maintained as a dispersion and prevented from agglomerating and resettling on the surface being cleaned. 6. SEQUESTRATION The process in which undesirable ions such as Ca++ or Mg++ and heavy metals are "tied up" or deactivated, thereby preventing them from reacting with material that normally would form insoluble products. The classic example is the hard water scum formed when soaps or synthetic detergents are used. The scum formed is the reaction between soap and the Ca++ or Mg++ ions in hard water. When the water is softened, the Ca++ and Mg++ ions are "tied up" or sequestered, preventing them from reacting. 7. MECHANICAL ACTION This is an extremely important factor in metal cleaning, since it can greatly increase the speed and efficiency of soil removal. Mechanical action can be accomplished by solution movement or movement of the work itself. Some types of solution agitation are air, mechanical, ultrasonic, spray and gas scrubbing from electrolytic cleaning. Air agitation is not as efficient as the other methods. CLASSES OF CLEANERS Cleaners fit into three groups - solvents, semi-aqueous and aqueous. 1. SOLVENT-BASED CLEANERS Both vapor degreasing and solvent immersion are being phased out or restricted as a result of safety concerns and environmental regulations. 2. SEMI-AQUEOUS OR DIPHASE CLEANERS
These are emulsions of water, organic solvents and emulsifiers. They have limited applications in plating lines, since they require secondary aqueous cleaning steps. 3. AQUEOUS CLEANERS Constitute the largest group and can themselves be fit into three categories: acid, neutral, and alkaline cleaners: a. Acid Cleaners Based on low concentrations of organic or inorganic acids such as citric and phosphoric. They are used when light metal oxides are present. The type of acid and additive package used depends on the basis metal, type of soil and subsequent processing. Applications for acid cleaners include buffed aluminum and brass where minimum or no attack of the surface is tolerated. b. Neutral cleaners (also called detergent cleaners) Neither acid nor alkaline based. They normally consist of sequestering agents, dispersants, buffering agents and wetting agents. High concentrations and temperatures make their use limited. c. Alkaline cleaners The workhorse of the industry. They are easy to use, relatively safe and easy to control. They are also more flexible and have broader operating limits than other cleaners. Alkaline cleaners are composed of chemicals that provide alkaline conditioning, saponification of fatty soils and soil suspension. General chemistries used in alkaline cleaners: Caustic alkalis - provide the highest level of alkalinity, with reasonably low costs. They are poor rinsers, and the powder forms attract moisture. They provide excellent alkalinity when used in electrocleaners. Carbonates - provide buffering, rinse well and soften water by precipitation. Depending on use, they provide a wide range of alkalinity. Carbonates can cause scaling in hard water areas if not properly mixed with other water- softening agents. Silicates - are close to the caustic alkalis in terms of alkalinity. They are good buffers, excellent emulsifiers and help in soil suspending. Silicates must be well rinsed prior to acid dips or pickling to prevent insoluble silicic acid gels from forming. Phosphates - provide water softening, mild alkalinity and some buffering. They are synergistic with the wetting agents, enhancing the overall cleaning effect. WETTING AGENTS (SURFACTANTS) Wetting agents are critical to the function of most cleaners. These organic materials possess the unique property of having water repelling (hydrophobic) and water attracting (hydrophilic) opposite ends of their molecules. This antagonistic effect allows for the penetration of these materials at the water-oil (organic contaminant-metal surface), interfaces, thereby facilitating removal and solubilization of various soils. Common surfactants used in cleaners are: Anionics - which provide good detergency. They are also good coupling agents for the other surfactants. Soap is an example of a naturally derived anionic surfactant. Anionic surfactants are generally high foamers. Nonionics - are synthetic materials and are good emulsifiers. In addition, they are used to dedust powered cleaners and for foam control in spray cleaners. The solubility of nonionic surfactants in
alkaline cleaning solutions decreases with increasing temperature and concentration. This is called the cloud point. When an alkaline cleaner containing nonionic surfactants is operated above the recommended temperature and concentration, the surfactant becomes water insoluble and begins to act like oil. In an immersion cleaner this is detrimental and can cause the loss of the surfactant through skimming devices. In a spray cleaner, low-cloud-point nonionic surfactants are often used as defoamers. Amphoterics - are similar to anionic surfactants when used in alkaline cleaners. They provide excellent stability in strongly alkaline liquids. Amphoteric surfactants must be well rinsed prior to any contact with acids, which will convert them to cationic film formers. Cationic - surfactants are film formers and are used in special alkaline cleaners and as inhibitors in acids. ALKALINE DESCALING Alkaline descaling is a special type of cleaning that is designed to remove light oxides and scale from ferrous substrates. The cleaners are based on high alkalinity in combination with complexing and chelating agents. Used at 168.8-185¡F, alkaline descalers are capable of dissolving light oxides and scale by immersion. One or more additional cleaning actions as described under Mechanism of Soil Removal are usually involved in soak cleaning applications. Removal of buffing compounds, for instance, requires good saponification, emulsification and deflocculation. On the other hand, if the emphasis is on separation and skimming of oils from cleaners, displacement, rather than emulsifying type cleaners are selected, along with chemicals that provide good wetting characteristics. METHODS OF SURFACE PREPARATION A total surface preparation process consists of one or several operations. They are carried out in a sequence designed to produce an optimum condition for the application of the intended coating. These steps are typically classified in accordance with their particular cleaning mechanism, as follows: • •
• •
MECHANICAL CLEANING (spray and ultrasonic) SOAK CLEANING ELECTROCLEANING ACID ACTIVATION
While some pretreatment processes call for soak cleaning followed by acid activation, others may require all or most of the above operations. 1. MECHANICAL CLEANING Mechanical cleaning in the context of surface preparation for metal coating encompasses spray and ultrasonic cleaning. Mass finishing is also used, but is described in detail in a separate QMF Guide. Mechanical cleaning relies in great part on mechanical energy in physically removing soils from the surface. To assist the removal process, detergents, alkalis or acids, deflocculants and so on are used. Heat also helps, although low to moderate solution temperatures are the norm. a. Spray Cleaning This is a very common cleaning technique, often used in dedicated spray machines, or built into stations within a plating line. Primps deliver cleaning solutions through spray nozzles at pressures varying from 20 to 200 psi. Some high-pressure systems can produce 800 psi or more. The cleaning solution is recirculated through a reservoir, which is usually fitted with heaters, oil skimmers, and level controllers.
Spray-cleaning solutions are formulated with low or non-foaming detergents. Since the cleaning action relies mostly on the spray-impact energy, the solution concentration is kept relatively low, as is the temperature. Spray cleaners are designed for specific applications, taking into account the basis metal being processed and types of soils to be removed.Mild acid or alkaline cleaners are used on nonferrous and highly finished metals, e.g., buffed brass and aluminum. Stronger cleaners, incorporating rust inhibitors, are used on ferrous metals, including steel alloys. Typical soils removed by spray cleaners are buffing compounds, heavy oils, drawing and stamping compounds. Parts that benefit from spray cleaning include cabinets and other geometric shapes with deep corners and recesses. For superior cleaning, turbojet nozzles are used inside an immersion tank. Parts may be cleaned in stationary positions or on conveyors moving across banks of spray nozzles positioned to reach all areas to be cleaned. Spray cleaning may be single or multiple stations followed by rinsing, inhibiting and drying or other surface-preparation steps. b. Ultrasonic Cleaning Ultrasonic cleaning takes place when small vacuum cavities caused by high-frequency sound waves traveling in a liquid implode against surfaces submerged in the liquid. Transducers attached to the sides or bottoms of cleaning tanks supply the ultrasonic waves to the cleaning solution. Temperatures above 140-160¡F should be avoided, as they tend to reduce the ultrasonic efficiency. When this technique is used with a water-detergent solution, the scrubbing action that results at the surface is effective in removing various contaminants, especially insoluble particles, smut, dirt, and soils that are difficult to remove by conventional wetting or emulsification. It does not, however, effectively remove oils, which is why it is used after degreasing. 2. SOAK CLEANING Cleaning by immersion is intended to remove the bulk of the soils, particularly those that have been mechanically deposited on the surface. Examples of such soils are cutting fluids, grease, fingerprints and buffing compounds. There are various types of soak cleaners designed for different cleaning mechanisms: •
•
•
By saponification of fatty acids such as those encountered in buffing compounds, certain lubricants and drawing compounds. Saponification relies on the level of alkalinity available in the cleaner and is promoted by high temperatures. By emulsification of soils, grease, fatty acids and their derivatives. This is accomplished through the use of specialty surfactant blends and certain alkalis such as silicates. By displacement of oils and grease, through the use of specialty surfactants. Displacement allows the oils to be temporarily tied to the surfactants at the metal surface, where they are lifted, then released at the surface of the solution. Effective skimmers must be used with such cleaners, to prevent drag out of released oils on the work. These cleaners usually require use of a longer tank, since they are easily saturated with oil.
3. ELECTROCLEANING Electrocleaning is an important step in metal-surface preparation. It is an electrolytic process, characterized by the use of direct current and a specially formulated electrolyte. The work being processed may be electrically connected as the anode, cathode, or both (alternately), depending on the application.
Although electrocleaning is a different and distinct method of surface cleaning, it should be considered an integral part of the preplate cycle. Electrocleaners as described here are alkaline, and will generally follow alkaline soak cleaners, and precede acid activation in the preplate cycle. Their basic function is to remove soils that could not be removed by simple soak cleaning or degreasing. Examples of such soils are as follows: •
•
•
Fingerprints, drawing compounds, and soils driven into surface porosity or applied under pressure. Finely divided particles such as polishing abrasives, metallic fines from grinding or metalworking operations, carbon and other alloying elements. Often these fine particulate matters, generally referred to as smut, may be held to the surface by simple mechanical forces, electrostatically, or in a thin oil or grease matrix. Metal oxidation products, the result of exposure to the atmosphere, or to a thermal process such as heat treatment, forging, welding, etc.
Although electrocleaning may not totally remove a particular type of soil, it conditions or modifies that soil for easier removal in the subsequent steps in the cycle. For instance, an adherent oil residue may be loosened enough to be lifted in the following rinse tank. Surface oxides may be reduced or oxidized to a more soluble form to be easily dissolved in the acid tank. a. Electrolysis of Electrocleaners As current is applied to an electrocleaner, the following electrochemical reactions take place, essentially electrolyzing water. The alkalis serve as the conductive medium. At the Anode:
4[OH]- --> 2H20 + 02 + 4e
At the Cathode:
4H20 + 4e --> 4[OH] + 2H2
As can be seen, twice as much hydrogen is liberated at the cathode as oxygen at the anode. b. Types of Electrocleaners Electrocleaners are classified on the basis of two main interrelated criteria: Polarity of the work in the tank. Type of substrate being treated. Anodic Electrocleaning The work is connected to the positive (anode) side of the rectifier. This process is also known as reverse electrocleaning, since the polarity is opposite to that in electroplating. As described under electrolysis, oxygen is liberated at the surface of the work (the anode) when current is applied. As the gas rises to the top, it creates a mechanical scrubbing action that loosens and lifts the soils. Two other phenomena also take place. As oxygen bubbles are formed on the surface, they coalesce and grow before they rise in continuous layers. It is believed that the static charge holding fine particles to the surface is released through the layers of bubbles, facilitating soil removal through the scrubbing action. This phenomenon also applies to the other modes of electrocleaning.
Solution Conductivity This in turn is a function of cleaner concentration and temperature at a given voltage. The higher the concentration and temperature, up to a practical level, the higher the conductivity and the amount of gassing and scrubbing action. Voltage Applied-Current increases with voltage, although the latter is kept to maximum values of 10-12 V. Higher values are known to cause "burning" or roughness on parts.Chemical effects, oxidation and drop in pH also take place at the anode surface. If excessive, the effect of oxidation can be seen on such metals as brass, zinc and silver, as they discolor, stain or etch. Special inhibited anodic electrocleaners are available for brass and zinc. When nickel is anodically treated, it will quickly passivate and prevent plating on its surface unless reactivated. A similar effect is experienced with stainless steels. Regular steels are not adversely affected by anodic electrocleaning, whereas high-carbon steels are more sensitive and require moderation. Alloys of lead, nickel and silver are attacked or tarnished by anodic electrocleaning. As oxygen is liberated at the anode, the net pH value tends to increase at the interface. This effect can be noticed on steel if an electrocleaner's alkalinity is too low by design or as a result of poor bath maintenance. The steel is more rapidly oxidized, and precipitated iron hydroxide forms on the surface. Parts exiting the tank will have a rusty or etched appearance, especially in highcurrent-density areas. The situation can be readily rectified by increasing the alkalinity of the bath or by reducing the current density below normal operating levels. Cathodic Electrocleaning The work is connected to the negative terminal (cathode) side of the rectifier. This is also known as direct electrocleaning. In this case, hydrogen is liberated at the cathode. Twice as much hydrogen as oxygen is generated. Consequently, more scrubbing action and cleaning ability are expected. Cathodic electrocleaning, however, has not found as widespread use in industry for two reasons: the concern about hydrogen embrittlement as a result of copious hydrogen release at the surface; and the risk of plate out of charged impurities from the solutions onto the cathodic surface. The latter may not be perceptible to the casual observer as the parts exit the tank, but leads to poor adhesion of subsequent electrodeposits. Contaminants leading to such adhesion failures are metallic fines, certain types of surfactants, colloids, metallic soaps and dragged-in hexavalent chrome. Cathodic cleaners, when kept clean and well maintained, are used for processing buffed brass, zinc and white metal, without tarnishing, and for electrocleaning nickel and high nickel steels without risking passivation. When used on steel and copper to take advantage of their superior scrub-clean action, a secondary anodic electrocleaner should follow, only if even for a few seconds. This step will deplate any impurities that may have deposited on the work by cathodic action. Periodic-Reverse Electrocleaning This method of electrocleaning of ferrous metals uses a combination of both anodic and cathodic cleaning. A periodic-reverse unit (known as P.R.) is installed on the rectifier's output. The PR unit has a switching mechanism that reverses the polarity at controlled and timed intervals. The work in the tank assumes alternating anodic and cathodic polarities for the specified cleaning time. A typical setting is 10 sec. cathodic - 10 sec. anodic. By changing the setting, more cathodic or anodic may be used to effect maximum cleaning. The unit can be programmed so that the last leg of the cycle is anodic before the time setting shuts off the rectifier. This ensures removal of any charged particles that may have plated on the work during cathodic cleaning. The continuous oxidation and reduction at the surface converts the oxides and scale on the parts to more soluble forms that are picked up by cyanide or chelating agents built into the cleaner formulation. PR cleaners eventually become saturated with dissolved iron oxides and must be replaced. Where practical, they can be regenerated by plating out the iron cathodically. PR.
cleaning is very efficient in descaling and derusting high strength and spring steels without the use of descaling acids in the cycle, thereby controlling hydrogen embrittlement. (ASTM B242 deals specifically with the standard practice recommended for the preparation of high carbon steel for electroplating to minimize hydrogen embrittlement). c. Operating Parameters and Process Considerations As explained earlier, electrolysis is the main driving process in electrocleaners. The amount of gassing responsible for the scrubbing action at the electrodes is a function of the amount of current passing through the cell. Therefore, these parameters controlling current should be considered: Solution Conductivity This in turn is a function of cleaner concentration and temperature at a given voltage. The higher the concentration and temperature, up to a practical level, the higher the conductivity and the amount of gassing and scrubbing action. Voltage Applied-Current increases with voltage, although the latter is kept to maximum values of 10-12 V. Higher values are known to cause "burning" or roughness on parts. Surface Area Being Cleaned The surface area being cleaned electrolytically and the amount of current applied result in a factor called current density (CD). It is measured in amperes per square foot (asf). Current density affects cleaning efficiency. Adequate recommended CD ranges for different basis metals are summarized in Table I. CD below these values produces light to marginal electrocleaning. Higher values generally lead to etching and roughness of the surface. Anode to cathode area ratios of 1:1 are adequate for most applications. The ratio is not critical as long as the specified current densities are maintained. TABLE I Recommended Electrocleaning Current Density-Rack Applications Amps/ft2
Polarity (A=Anodic, C=Cathodic)
Steel, low-carbon
47-93
A,C
Steel high strength
28-47
A
Stainless steels
47-74
C
Copper
47-74
A, C
Brass
19-37
A, C (anodic inhibited)
Zinc diecast
19-37
A, C (anodic inhibited)
Nickel and its alloys
19-28
C
Lead and its alloys
47-74
C
Silver and its alloys
19-28
C
*In barrel applications, a fraction of the above C.D. values should be expected.
Selection and Use of Electrocleaners - There are general considerations in the selection and proper use of electrocleaners: Formulation - Several proprietary formulations are available, covering a wide range of
applications. These formulations should provide the following properties: A suitable degree of alkalinity for the metal processed, e.g., high alkalinity for steel, lower for zinc and brass. A proper ratio of hydroxide to silicate to prevent insoluble silicate films from adhering to the work and affecting plate adhesion. TABLE II Typical Operating Conditions for Electrocleaners Steel and Copper
Brass
Zinc
Ni and Cr Alloys
Alkalinity, g/L (as NaOH)
50-100
15-20
15-20
30-60
Temperature °F
140-195
120-160
120-160
120-175
Time, seconds
1-5
1-3
1-3
1-5
The above are typical average operating conditions. Suppliers of proprietary electrocleaners usually specify optimum operating parameters for specific applications. Alkalinity expressed as NaOH may represent from 20 to 80% of the total product formulation. Equipment Maintenance and Operation-Corrugated or mesh steel can be used as anodes or cathodes, to provide optimum surface area and solution circulation. Periodic cleaning of the anode/cathode is necessary to remove plated-on smut, oxides and other charged particles. Using the tank as the anode or cathode is not recommended, as it leads to uneven current distribution and provides a source of stray current. Many electrocleaning problems, such as under- and overcleaning, have been traced back to such a practice. Solid polypropylene or plastic-lined steel tanks are recommended for alkaline electrocleaners. They should be fitted with steel, stainless steel or Teflon heaters. Recirculating pumps are recommended to provide solution recirculation, overall homogeneity, and to prevent stratification. It should be noted that solution inlets and outlets must be located at two opposite diagonal top and bottom corners of the tank for efficient solution movement. TABLE III Most Common Electrocleaning Problems Problem
Probable Causes
Etching, tarnishing
Too high C.D. Cleaner non-inhibited for nonferrous metals Wrong polarity Temperature too high for nonferrous Excessive oil in solution
Roughness
Cleaner too weak, causing burns on steel Excessive C.D. In anodic cleaning: pulling alloying elements to the surface In cathodic cleaning: depositing charged particles and smut on surface Incomplete rinsing of electrocleaner
Haze under plate
Cleaner temperature too high, causing dried-on film. Cleaner too weak Incomplete rinsing after cleaning Inefficient soak or precleaning prior to electrocleaning Hexavalent chrome contamination
Poor adhesion,
Cleaner too weak
blisters, pitting of plate
C.D. too low or too high Cleaning time too low or too high Reverse of intended polarity Hexavalent chrome contamination Insufficient rinsing after cleaning Excessive oil, grease in cleaner
4. ACID ACTIVATION Acid activation is used to activate the surface of a metal after alkaline spray, soak, or electrocleaning. As such, the acid neutralizes residual alkalinity as well as objectionable oxides, and exposes an active, readily plateable surface. This process is used to remove chemically formed soils from the metal surface. Typically these soils are metal oxides and their salts. They form as a result of exposure to high temperature, or to the atmosphere. Examples are heat-treating oxides and scales, forging and casting scales, rust, and simple oxidation. Although some of these oxides are plateable, they lead to poor plate adhesion and corrosion failures. Oxide removal is generally carried out by acid treatment. The acids are selected according to the metal being processed, so that limited or no attack to the basis metal occurs. There are three major types of acid activators: Organic and Mild Mineral Acids - Examples are gluconic, phosphoric, and citric acids. These are typically used to activate highly polished or sensitive metals such as aluminum, brass and zinc diecastings, and some polished steels.Surfactants and wetting agents may be used to reduce the surface tension and improve acid action. Mineral Acids - Hydrochloric and sulfuric acids are examples of strong mineral acids. These are generally used to descale and activate steel and some stainless steels. Nitric acid by itself or in combination with sulfuric and hydrofluoric is used to desmut and activate aluminum and its alloys. Fluoboric and hydrochloric are used on leaded steels and leaded brass. Combinations of nitric, sulfuric and hydrochloric are sometimes used in descaling and bright dipping copper and brass, although they generate heavy nitrogen oxide fumes and must be handled with extreme care. Special stable surfactants and inhibitors may be used with these acids to disperse oil or grease residue, to improve wetting and surface activation, and for minimizing attack of the basis metal. Acid Salts - Acid salts are typically of the sulfate type, based on partially neutralized sulfuric acid. The remaining hydrogen ion in the salt provides sufficient acidity to activate metal surfaces. Advantages of acid salts include safer handling and the flexibility to compound them with accelerators, surfactants and inhibitors to produce a wider range of specialty activators. Compound acid salt may, for example, include fluorides and inhibitors that are very effective on scale and smut removal from ferrous metals, yet inhibit attack of basis metal. They are generally non-fuming and less corrosive to plant equipment. The use of acid activators must follow careful considerations. The selected acid must not aggressively attack the metal being cleaned, as this may give rise to smut formation, pitting and roughness in the subsequent coating. The type of acid used also must be compatible with the
plating process. Hydrochloric acid, for instance, must not be used prior to acid copper plating. Chloride contamination can seriously affect performance of that plating bath. Likewise, sulfuric acid is not recommended for lead or leaded alloys, as it might form insoluble lead sulfate on the surfaces of parts. Use of strong mineral, uninhibited acids should be restricted on spring- and high-carbon steels, as the reaction with such acids leads to hydrogen embrittlement. Acid activators in preplate cycles may be used by simple dip or immersion, or electrolytically, depending on the metal being treated. Generally electrolytic acid activation is used on ferrous and stainless steel alloys. The parts are usually connected cathodically (negatively charged) in the tank. Lead or graphite anodes are used. Certain metals such as stainless steels are anodically (positively) charged while acid activated to provide superior adhesion of the plated coating. Nickel and nickel plated parts are acid activated electrolytically to remove the passive film prior to further plating. In this case, the nickel is connected cathodically. The passive film, nickel oxide, is readily reduced at the cathode. Specialized publications and recommendations of suppliers are good sources for information on the safe and efficient use of acid activators. SURFACE PREPARATION OF ALUMINUM AND ITS ALLOYS Surface preparation of aluminum for plating is different from that of ferrous and other metals. The difference is caused by the high degree of aluminum reactivity. On exposure to air, an oxide layer forms rapidly and covers the surface of aluminum. This layer prevents direct bonding with the basis metal. Another problem is the difference in microstructure between different aluminum alloys and the variations brought about by thermal treatments of the same alloys. The result is a range of electrochemical properties requiring different pretreatment and activation techniques. As with other metal substrates, surface soils such as oils, grease, buffing compound, fingerprints, etc. also may be present and must be removed. A general cycle usually followed to pretreat aluminum is as follows: 1. 2. 3. 4.
DEGREASING. OXIDE REMOVAL. SURFACE CONDITIONING TO PREVENT REOXIDATION. PLATE.
1. DEGREASING The purpose of degreasing is to remove mechanically deposited soils on the surface, e.g., stamping oils, buffing compounds, fingerprints, etc. Generally, any attack of the surface at this point would be uneven, resulting in a blotchy surface condition. The degreasing step, therefore, should be designed to remove these soils without attacking the basis metal. Mild alkalis, detergents, and solvents applied with or without ultrasonics are used for this purpose. Alkaline non-etch cleaners are widely used in this step. These cleaners may or may not contain silicates as part of their composition. Silicates are excellent degreasing agents, but at high concentration and temperature will form insoluble aluminum silicate films on the surface. To be removed effectively, these films require the use of fluoride additives in subsequent acid desmutters. Silicate-free non-etch cleaners rely on good surfactant and deflocculating agents to degrease the surface without leaving inhibiting films. Some recent developments have made it possible to combine a high degree of detergency with simultaneous etching properties. These formulations are limited to removal of light oils, and must be carefully monitored, since the etch rate may be affected by oil loading in the solution.
2. OXIDE REMOVAL As explained earlier, an oxide layer is ever present on aluminum surfaces. The layer's thickness and hardness depends on alloying elements, manufacturing techniques and heat treatment. Removal of the oxide layer is a critical step in the preplate cycle. Oxide removal can be achieved by various acid and alkaline treatments. Highly alkaline solutions containing sodium hydroxide offer a high rate of attack of aluminum and are very effective in oxide removal. Dissolved aluminum remains in solution as aluminate as long as sufficient free alkalinity is maintained in the bath. A number of additives are used to control etch rate, etch pattern or grain size, and to prevent the formation of hydrated aluminum oxide scale around heaters and tank walls. Removal of oxides by alkaline etching, however, leaves behind a residue of alloying or dispersed elements that are not alkaline soluble. This residue, described as smut, may contain silicon, copper, magnesium, etc. depending on the alloy composition. Therefore a desmutting step follows alkaline etching. Acid etchants are also available. They are used where aggressive alkaline etching is not desirable, such as on highly polished or buffed surfaces. Acid etchants leave less smut on the surface. Desmutters are selected to dissolve the elements comprising the smut, with little or no further attack of the aluminum. Table IV illustrates commonly used desmutters. These materials are generally acid-based solutions with oxidizing properties. Additives (such as sulfates, fluorides, and chlorides) are used, depending on the elements to be removed. Effective oxide removal, however, may not always be achieved by the conventional sequence of degrease, etch, and desmut. Heat-treated alloys that have been subjected to mechanical, cold or hot working such as extrusion, forming, or stamping may develop a "glazed oxide" layer. This is a hard oxide that resists alkaline etching and remains on the surface through the desmutting step, usually leading to adhesion failure when plated. Removal of such oxides can be accomplished by mechanical removal (shot blasting or mass finishing), or chemically-by deoxidizing the surface before alkaline etching. 3. SURFACE CONDITIONING After desmutting, it is necessary to prevent the reoxidation of aluminum in order to obtain optimum bonding of the electroplate on the aluminum surface. A number of metals can be deposited on the aluminum immediately upon leaving the desmutted surface. They are deposited by electroless immersion, instantly forming a protective barrier, e.g., zinc, tin, copper, nickel, etc. The most widely used for this purpose is zinc, deposited from a strong zincate solution. Although simple zincates are adequate for coating high grade aluminum, they are not as effective on certain alloys and castings. Newer alloy zincates containing complexed zinc and other metals have proven far superior on all alloys. The improved performance is a result of the controlled growth of the film and its chemical composition. The alloy zincate solution is designed to induce rapid nucleation and slow crystal growth on the surface. The result is a more uniform layer with excellent adhesion. The modified alloy zincate film is denser, finer and readily plateable, as long as the plating baths are not excessively acid or alkaline. The solution density is much lower than that of standard zincates, allowing for easier and faster rinsing. Alloys high in magnesium generally require the double zincate process. During the first zincate step, surface oxide is dissolved and replaced by the immersion zinc layer. This layer is then stripped in 50% nitric acid or other proprietary products, and a second zincate layer is applied.
Additional nucleation sites develop, resulting in a more uniform, even and adherent zinc alloy film. The latter prevents reoxidation of the surface, making transfer times prior to plating less critical. Another feature of alloy zincates is their capability to protect edges and sharp corners of parts processed in barrels or bulk from being worn, a common problem leading to blisters in barrel plating. TABLE IV Typical Desmutting and Acid-Deoxidizing Solution For 1XXX and 2XXX Alloys Nitric acid
30-70% v/v
Water
Balance
For 2XXX and 4XXX Containing Silicon and Most Castings Nitric acid
30-50% v/v
Fluoride
60-100 g/L
Water
Balance
For 5XXX Containing Magnesium (May also be used as all-purpose desmutter) Nitric acid
30-50% v/v
Sulfuric acid
20-30%
Fluoride
60-100 g/L
Water
Balance
For Copper Magnesium Alloys (Fluoride may be added for silicon-containing alloys) Sulfuric
5-10% v/v
Hydrogen Peroxide
3-5%
Water
Balance
Proprietary, nonfuming, stable-acid desmutters are also available for most applications. TABLE V Typical Preparation Cycles 1. For Simple Alloys and Castings: Non-etch soak. Etch. Desmut. Zincate. Plate. 2. For Heat Treated Alloys: Non-etch soak. Deoxidize. Etch clean. Desmut. Zincate.
Plate.
3. For Magnesium and Heat Treated Alloys: Non-etch soak. Etch. Desmut. Zincate. Strip (50% nitric acid). Zincate. Plate. Rinsing The chemistries of the different steps could vary substantially from tank to tank in a given surfacepreparation process. Types and degrees of alkalinity or acidity, surfactants, etc. may not be compatible among the different solutions. Such cross contamination can create a general drop in cleaning efficiency and cause objectionable residues to form on the surface being prepared. Silicate residue, for example, in contact with acids will precipitate insoluble silica acid on the metal surface, leading to haze, pitting and loss of adhesion when parts are plated. Rinsing is therefore considered an important and integral part of the surface-preparation process. Adequate rinsing should provide a flow rate adequate to remove residues from the previous step within the given rinse time, taking into account the extent that a maximum level of that residue is acceptable. Flow controllers, conductivity meters and counter-current-flow rinse techniques are tools used to optimize quantity and quality of water used. Rinse-water temperature is a factor often ignored in preplate cycles. Temperatures below 60¡F are inadequate in rinsing off alkalis and surfactant residues. This is a problem often encountered in cold weather, with incoming rinse water at 40-50°F. Optimum temperature for efficient solubility of alkaline cleaners and salts is 70-90°F. Higher t emperatures are effective but may induce flash rusting on steel. Air agitation and spray rinsing greatly improve rinsing efficiency and are often used where feasible. Reclaimed water for reuse in rinsing is generally acceptable in preplate cycles as long as the residuals are compatible with the different chemistries used in the process. A fraction of fresh water makeup is used to prevent an increase in residual content. Process Control Control of cleaners is usually done by titration of the acid or alkali contents. Maintenance additions will replenish the basic constituents as well as surfactants and other components included in the formulation. Suppliers of proprietary cleaners provide complete control procedures and kits that allow the user the means to control and maintain the solutions at optimum operating conditions. Although the essential components can be maintained, contaminants build up and eventually interfere with the proper performance of the bath. Oils and grease, if not adequately removed in the preceding soak cleaners, may result in water breaks on the surfaces emerging from the electrocleaner or acid tank. Grease etch is also a result of such a buildup. It shows as jagged etch spots after plating. It is caused by uneven acid activation or electrocleaning around nonwetted spots on the surface. Hexavalent chromate trapped in cracked rack coating and dragged into the electrocleaner is another source of contamination. This leads to drastically reduced cleaning and haze under nickel plate. Stripping chrome plated parts for rework, using the process electrocleaner, has a similar effect. Hexavalent chrome contamination can be detected readily, as the cleaner turns yellow-orange and foaming seems to subside. Compatible chrome reducers are used to counter this effect. They reduce the chrome to trivalent
if the cleaner is not heavily chelated, and allow mostof it to precipitate as the hydroxide. The solution color changes to light green, indicating that the reduction process has taken place. Cleanliness Criteria There are many references in the literature about this subject. Most, however, deal with specific applications and therefore are not universal. These include UV detection of oil residue, the oil being of special UV- sensitive composition, solvent extraction, drying and weighing of extract, etc.The most commonly used method is the Water Break Test. The work is inspected after the final cleaning step for water breaks. These are evidenced by a break in the continuous film of water on the surface, after the work is removed from the last rinse station. The water-break test is best done after acid dipping, since an alkaline film may give a false water-break-free appearance. The presence of a water break signifies the presence of a hydrophobic, water-repelling film still adhering to the surface, causing a non-wetted condition. Such a residue would ultimately lead to poor adhesion of the plate. ASTM B320-60, another method of measuring cleanliness, requires wiping the surface with a clean white cloth before the plating step. This will reveal whether smut or other soil residues remain on the surface. Careful inspection of parts after plating is also used as a test for cleanliness. Haze or stain pattern under the plate is usually indicative of incomplete cleaning and surface preparation. ASTM B-322, Section 8.1, provides additional details for the Water Break Test. A specific drainage time of about 30 seconds is required after rinsing, before observation of the film. Adhesion Testing Since surface preparation has a direct impact on adhesion of the plate, adhesion testing is often performed to measure the adequacy of the preplate process. There are several adhesion tests performed as a form of production control. The choice of a particular test is limited by the type of part and its intended use. For example, if a part is expected to be exposed to high temperature, a heat-and-quench test would be relevant. Bending would be an adequate test for post-plate forming operations. ASTM B-571-91 describes the following tests in detail. One or more may be specified for a particular application. In all tests, the coating is expected to remain intact, with no signs of peeling or flaking off the substrate. a. Bend Test Bending of parts at a 90°angle around a mandrel, o r back and forth through 180°until failure of the basis metal occurs. Cracks in the metal without lifting of the deposit do not constitute a failure. b. Chisel-Knife Test An attempt is made to undercut and lift the coating with a sharp chisel or knife. Not recommended for soft or thin deposits. c. Burnish Test Rubbing and applying pressure on the plate with a smooth-ended tool, producing a burnishing effect with no peeling or blistering. Not satisfactory for thick deposits. d. File Test The cross section of the part is filed with a coarse file at a 45°angle so that the substrate is file d away, leaving the edge of the plate protruding. The plate should not lift or peel when pulled. e. Grind-Saw Test Grind or saw the plated article in the direction that favors separation of the plate. f. Heat-Quench Test Articles are heated in an inert or reducing atmosphere, then quenched in water. The
temperatures for different basis and plated metals are specified in ASTM B57. Appearance of blisters may occur. If the deposit cannot be peeled off the substrate in areas around the blisters, it is not an adhesion failure. g. Impact Test A hammer or an impact device is used to strike and deform the plated part. h. Peel Test A strip of steel or brass is soldered or secured by a heat cured adhesive to the plate. The strip is pulled at 90°to the surface. i. Push Test A blind hold is drilled through the substrate on one side of the part, short of reaching the coating on the other side. A punch is pushed through the hole against the supported part until a button is punched out. No exfoliation or peeling in the punched area should occur. j. Scribe-Grid Test This is a commonly used test in which the surface is scribed with a sharp blade in a grid pattern. A special adhesive tape is applied to the scribed area, then pulled off rapidly. No peeling should occur. TABLE VI Typical Cleaning Cycles COLD ROLLED STEEL Substrate
Alkalinity gm/l of NaOH
Soak clean highSoak clean high
80-100
Electroclean
50-100
Typical Cleaner Condition
20-100 ASF Anodic or Cathodic followed by Anodic
Rinse Acid Activate
n/a
no current or 15-25 ASF Cathodic
Rinse Electroclean
50-100
20-100 ASF Anodic
Rinse Mild acid dip plate
n/a
3-5% HCI
STAINLESS STEEL Compound remover
5-15
used if parts are polished soak or Ultrasonic
Soak clean
20-100
8 oz/gal, 4 min, 160¡F
Electroclean
50-100
20-50 ASF Anodic
Acid activate or Plate
50%Hcl
15-25 ASF Anodic
ZINC DIE CAST Compound remover
5-15
Rinse
Soak clean moderate Electroclean
Soak or Ultrasonic
low, 5-15 10-20
10-15 ASF Anodic
Rinse Acid dip plate
n/a
COPPER Soak clean high
20-100
Electroclean
20-100
20-50 Anodic
Rinse Acid activate
n/a
BRASS Soak clean moderate
5-15
Electroclean
10-20
10-15 ASF Anodic or Anodic
Rinse Acid dip
n/a
TUNGSTEN Soak clean high
20-100
Acid dip
n/a
Rinse Anodic etch plate
20-100
100-300 ASF Anodic
anodic TYPICAL ALKALINITY RANGE High
20-100
Moderate
10-20
Low
5-15 (May not contain free Hydroxides)
Temperature and time vary depending on supplier recommendations and process used. LIQUID CLEANERS Traditional cleaning products were dry powdered materials. Though liquid cleaners were available as early as the 1970's, they were not accepted, for a variety of reasons. The most commonly cited reasons were that liquid cleaners were not concentrated, as were powders, had narrower operating ranges, and that the user was "paying to ship water." Advances in surfactant technology and the wide use of potassium salts have enabled chemical manufacturers to overcome these perceived problems. Put another way, liquid cleaners are not powdered cleaners pre-dissolved in water. Liquid cleaners are chemically different than powders in the use of potassium salts, safe solvents, surfactants, and a lack of carbonates and other fillers. A final factor in the acceptance of liquid cleaners is the inherent advantages they imply to the user, some of which are: • •
•
•
Improved safety, no risk of splashback. Automatic control and replenishment-hands-off operation, consistent bath chemistry, resulting in fewer defects. They are more suitable for SPC. Less down time for make up - less sludge generated, less demand on the wastetreatment system. Effective and economical cleaning.
Cleaner filtration is gaining in popularity, with the aim of prolonging the bath life between dumps. Several filtration techniques have been proposed, ranging from simple bag filtration
to complete systems of soil skimmers, coalescers and ultrafiltration. Since the cost of such systems varies appreciably, a feasibility study must be undertaken before adopting a particular system. In general, however, it has been reported that any type of filtration does increase the bath life by at least 20%, and up to five times or more. Recent developments in preplate products promote the use of environmentally acceptable chemistries. Substituting effective biodegradable surfactants for hard chelates and use of safer solvents and emulsifiers in place of chlorinated solvents are examples. Emphasis on simplified water treatment is another aspect of newer cleaning technologies. Recent developments allow oil separation from process cleaner. While water treatment is made easier, the cleaner solution life is extended and its performance maintained at optimum longer. Separated oils are reclaimed and reused. SAFETY AND HANDLING The majority of people who operate cleaner and acid tanks pay only minor attention to this extremely important subject. Many serious accidents, such as chemical burns, respiratory problems and eye injuries can be avoided. This information is given to the user by the suppliers, including specific instructions for handling process chemicals. The following safety tips should help inexperienced personnel to avoid injuries. •
•
•
•
•
•
•
• •
When making up acid-dip tanks, never add water to the concentrated acid, always add the acid to the water, with constant stirring to avoid spattering. Sulfuric acid is a good example. It might react violently if not handled properly. Never add dry cleaner (alkaline) directly to a hot tank above 120°F. Violent eruptions can occur if large quantities of dry cleaner are added directly to the tank. Instead, add dry product slowly, with continuous agitation. Avoid breathing fumes from acids and cleaners. Otherwise respiratory problems can occur. Avoid skin and eye contract when handling chemicals. Wear protective clothes, gloves, eye goggles and other protective equipment. A fume hood should be used when an analytical method involves the liberation of hazardous fumes. In case of injury, contact a physician immediately. Give first aid and see MSDS sheet. If a spill occurs, clean up the spill as soon as possible and prevent the spill from any contact with other chemicals, to prevent reaction. Maintain your own safety first, secure the area and get help. Do not allow personnel to work over hot solution without adequate precautions. Never work alone in an area where chemicals are in use. Provide professional training to those involved in handling chemicals. Check appropriate MSDS sheets prior to the handling or use of all chemicals.
Good laboratory safety practice requires that incompatible chemicals be stored, transported, and disposed of in ways that will prevent their coming together in the event of an accident. Tanks should be properly labeled and identified by type of chemical used, hazard classification and safety information. The preceding material has been reproduced from the MFSA Quality Metal Finishing Guide,Volume 1, No. 1-P "Chemical Surface Preparation for Electroplated Metallic Coatings"
