Designation: B177/B177M − 11
Endorsed by American Electroplaters’ Society Endorsed by National Association of Metal Finishers
Standard Guide for
Engineering Chromium Electroplating 1 This standard is issued under the fixed designation B177/B177M; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval. This standard has been approved for use by agencies of the Department of Defense.
B244 Test Method for Measurement of Thickness of Anodic B244 Test Coating Coat ingss on Alu Alumin minum um and of Oth Other er Non Noncon conduc ductive tive Coatin Coa tings gs on No Nonma nmagn gneti eticc Bas Basis is Me Metal talss wit with h Edd EddyyCurrent Instruments B253 Guide B253 Guide for Preparation of Aluminum Alloys for Electroplating B254 Pr Pract actice ice fo forr Pr Prep epar arati ation on of an and d El Elect ectro ropl plat atin ing g on Stainlesss Steel Stainles B281 Practice for Preparation of Copper and Copper-Base B281 Alloys for Electroplating and Conversion Coatings B320 Practice B320 Practice for Preparation of Iron Castings for Electroplating B322 Guide B322 Guide for Cleaning Metals Prior to Electroplating B481 Pra Practic cticee for Pre Prepar paratio ation n of Ti Titan tanium ium and Ti Titan tanium ium Alloys for Electro Electroplating plating B487 Test Meth Method od for Mea Measur suremen ementt of Met Metal al and Oxi Oxide de Coating Coat ing Thi Thickn ckness ess by Micr Microsc oscopi opical cal Exa Examina mination tion of Cross Sectio Section n B499 Test B499 Test Method for Measurement of Coating Thicknesses by th thee Ma Magn gnet etic ic Me Meth thod od:: No Nonm nmag agne netic tic Co Coat atin ings gs on Magnetic Basis Metals B504 Te B504 Test st Metho Method d for Measurement Measurement of Thickness of Metallic Coatings by the Coulometric Method B507 Practice B507 Practice for Design of Articles to Be Electroplated on Racks B558 Practice B558 Practice for Preparation of Nickel Alloys for Electroplating B568 Test B568 Test Method for Measurement of Coating Thickness by X-Ray Spectrometry B571 Practice B571 Practice for Qualitative Adhesion Testing of Metallic Coatings B578 Test B578 Test Method for Microhardness of Electroplated Coatings B602 Test Method for Attribute Sampling of Metallic and B602 Inorganic Coatings B630 Practice B630 Practice for Prepa Preparation ration of Chrom Chromium ium for Electro Electroplatplating with Chromium B650 Specification B650 Specification for Electrodeposited Engineering Chromium Coatings on Ferrous Substrates B697 Guide B697 Guide for Selection of Sampling Plans for Inspection of Electrodeposited Metallic and Inorganic Coatings
1. Sco Scope pe 1.1 This guide provides provides information information about the deposition deposition of chromium chromi um on ste steel el for engineeri engineering ng use uses. s. Thi Thiss is som sometim etimes es called “functional” “functional” or “hard “hard”” chrom chromium ium and is usually applie applied d directly to the basis metal and is usually thicker than decorative deposits. 1.2 The values stated in either SI units or inch-pound inch-pound units are to be regarded separately as standard. The values stated in each system may not be exa exact ct equ equiva ivalent lents; s; the theref refore ore,, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard. 1.3 This guide is not intended as a standardized standardized procedure, procedure, but as a gui guide de for obtainin obtaining g smo smooth oth,, adh adhere erent nt coa coating tingss of chromium of a desired thickness while retaining the required physical and mechanical properties of the base metals. Specified chromium electrodeposits on ferrous surfaces are defined in Specifi Specification cation B650 B650.. standard d doe doess not purport purport to add addre ress ss all of the 1.4 This standar safet sa fetyy co conc ncer erns ns,, if an anyy, as asso socia ciated ted wi with th it itss us use. e. It is th thee responsibility of the user of this standard to establish appro priate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referen Referenced ced Documents Documents 2.1 ASTM Standards:2 B183 Pra Practic cticee for Pre Prepar paratio ation n of Low Low-Ca -Carbo rbon n Ste Steel el for Electroplating B242 Guide B242 Guide for Preparation of High-Carbon Steel for Electroplating 1
This guide is under the jurisdiction of ASTM Committee B08 Committee B08 on Metallic and Inorganic Coati Inorganic Coatings ngs and is the direc directt respon responsibili sibility ty of Subco Subcommitte mmitteee B08.03 on Engineering Coatings. Current Curre nt editio edition n approv approved ed June 1, 2011 2011June June 1, 201 2011. 1. Publi Published shed June 2011 2011.. Origin Ori ginall ally y app approv roved ed in 195 1955. 5. Las Lastt pre previo vious us edi editio tion n app approv roved ed in 200 2006 6 as B177 – 01(2006)e01. DOI: 10.1520/B0177_B0177M-11. 2 For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at
[email protected]. For Annual Book of ASTM Standards volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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B177/B177M − 11 B762 Test Method of Variable ariabless Samplin Sampling g of Metallic and Inorganic Coatings B849 Specification for Pre-Treatments of Iron or Steel for B849 Reducing Risk of Hydrogen Embrittlement B850 Guide B850 Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement B851 Specification B851 Specification for Automated Controlled Shot Peening of Metallic Articles Prior to Nickel Nickel,, Autocatalytic Autocatalytic Nickel Nickel,, or Chromium Plating, or as Final Finish F519 Test F519 Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments 2.2 Military Standard:3 MIL-S-13165B Shot MIL-S-13165B Shot Peening of Metal Parts
at which the whole of each part attains the specified temperature. This stress relief is essential if hydrogen embrittlement from subsequent operations is to be avoided. 3.4.2 Parts having surface-harde surface-hardened ned areas that would suffer suffer an unacceptable reduction in hardness by baking in accordance with Specification B849 Specification B849 may may be baked at a lower temperature but not less than 130°C for a minimum period of 8 h. Shorter times at higher temperatures may be used, if the resulting loss in surface hardness is acceptable. 3.5 Oxidation— All All possible precautions should be taken to preven prev entt ox oxid idat atio ion n of th thee me metal tal su surf rface ace be betw tween een th thee fin final al operations of mechanical preparation and electroplating, particularly ticular ly with steel substrates. substrates. Materials such as alumin aluminum um and titanium have an inherent oxide film on the surface that can only be removed or minimized just prior to the electroplating process (see 6.1.1 and and 6.1.2 6.1.2). ). When conditions are especially unfavorable, definite steps must be taken to meet this important requirement, including storage in a noncorrosive environment, or the use of a suitable coating to exclude air and moisture.
3. Subst Substrate ratess 3.1 Eng Engine ineeri ering ng chr chromi omium um may be pla plated ted dir directl ectly y to the surfacee of a num surfac number ber of com common monly ly use used d eng engine ineeri ering ng meta metals ls such suc h as alum aluminu inum, m, nic nickel kel allo alloys, ys, cast iro iron, n, stee steels, ls, cop copper per,, copper alloys, and titanium. The bond strengths of the chromium miu m va vari ries es wi with th me metal tallic lic su subs bstr trate ate.. Ne Neve vert rthe hele less ss,, if th thee procedures cited in the appropriate references are followed, the bond strength is such that grinding and honing can be conducted without delamination of the coating.
4. Racks and Anode Anodess 4.1 Stee Steel, l, cast iron, iron, and stainless stainless steel parts to be ele electro ctro-plated may be racked at any convenient stage in the preparatory process but preferably prior to the final cleaning and etching. Alumin Alu minum, um, tita titaniu nium, m, and certain nickel nickel allo alloys ys may nee need d to have cleaning and etching operations done before racking due to entrapment of cleaning and etching solutions in the plating rack rac k whi which ch can result in adh adhesio esion n fail failure uress due to see seepag pagee during chromium electroplating.
3.2 Smoothness— The The smoothness of the material surface to be electroplated should be adequate to meet the requirements of the fini finishe shed d pro produc duct. t. Chr Chromi omium um elec electro trodep deposi osits ts do not exhibit leveling, and consequently the surface roughness of the electrodeposit will always be greater than that of the substrate. Any mechanical operations that can result in grinding checks or glazing of the metal are detrimental and should be eliminated. The required surface smoothness may be obtained by suitablee chemica suitabl chemical, l, mechan mechanical, ical, or electro electrochemic chemical al proce procedures dures.. Depend Dep ending ing upo upon n the thic thickne kness ss of the elec electro trodep deposi ositt and the smooth smo othnes nesss req requir uired ed of the elec electro trodep deposi osit, t, gri grindi nding ng of the electrodeposit may be required.
4.2 See Practic Practicee B507 B507 for for guidance on rack design, but note that while the general principles of good racking as used in other electroplating processes apply, the use of much higher current densities and the desirability of securing coatings of unifor uni form m thic thickne kness ss and qua quality lity on des desired ired areas req requir uiree rac rack k construction designs and methods that are much more exacting. The design of racks for chromium electroplating on the various base metals previ previously ously mentioned mentioned for functional functional use should provide for the following to the greatest possible extent. 4.2.1 There must be suf suffficient current-carryin current-carrying g capacity of both cathode and anode circuits to all parts of the rack. 4.2.2 There must must be positive electrical electrical contact to the parts parts to be electroplated, to the anodes, and to the tank contact bus bars. 4.2.3 4.2 .3 The There re mus mustt be uni unifor form m cur curren rentt dis distri tribut bution ion on the parts to be electroplated. This often requires anodes of special shap sh apes es co conf nfor ormin ming g to th thee sh shap apee of the pa part rt or ar area ea to be electroplated. 4.2.4 It may be necess necessary ary to use thieves, robbers, robbers, or guards, which are auxiliary metallic conductors placed near points of abnorm abn ormally ally hig high h cur curren rentt den density sity to attr attract act the cur curren rentt awa away y from fr om su such ch po poin ints ts;; an and d sh shie ield lds, s, wh which ich ar aree pa part rtss ma made de of nonconductive materials and placed to disperse the current in areas where it tends to concentrate unduly. 4.2.5 It is important important to protect areas that are to remain free of any chromium electroplate by the use of masks made of rigid, nonconductive materials placed against the substrate, or stop-of stopoffs, fs, which are especi especially ally compo compounded unded nonco nonconducti nductive ve tapes, waxes, lacquers, or plastics for the protection of such
Considerati derations— ons— Crac 3.3 Fatigue Consi C racki king ng th that at ca can n oc occu curr in chromium electrodeposits either as a function of the plating bath chemistry or the plating conditions, or both, or as a result of grinding of the electrodeposit can lead to a reduction in the fati fa tigu guee li life fe of th thee el elec ectr trop opla late ted d pa part rt.. If th this is is a de desi sign gn consid con sidera eratio tion, n, the use of mec mechan hanical ical meth methods ods suc such h as sho shott peening (see Specification B851 Specification B851 or or MIL-S-13165C, or both) or autofrettage to compressively stress the surface can increase the fat fatigu iguee str streng ength. th. This sho should uld be don donee afte afterr any stressstressrelieving relievi ng heat treatme treatment. nt.
3.4 High-Strength Steel Stress Relief: 3.4.1 3.4 .1 All ste steel el par parts ts hav having ing an ult ultimat imatee ten tensile sile strength strength of 1000 MPa [150 000 psi, approximately 32 HRC] or greater, which may contain residual stress caused by various fabrication operations such as machining, grinding, straightening, or cold-forming, usually will require one of the stress relief bakes prescribed prescr ibed in Specifi Specification cation B849 B849 prior prior to electroplating. In all cases, the duration of the bake shall commence from the time
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B177/B177M − 11 substrates. Lead and aluminum tapes will provide a sharp line of dem demarca arcatio tion n bet betwee ween n coa coated ted and uncoated uncoated are areas as wit with h a minimum of buildup. 4.2.6 Plugs (conducting and nonconducting) nonconducting) may be used in holes hol es not req requir uiring ing elec electro tropla plating ting to pro produc ducee a sha sharp rp edg edgee without grooves around the periphery of the holes. 4.2. 4. 2.7 7 It is ve very ry imp impor ortan tantt to re reme memb mber er th that at im impr prop oper erly ly applied stop-off materials or poorly designed racks can entrap acids that can cause corrosion corrosion of the basis material or contamination of the solutions used in subsequent operations, or both. 4.2.8 4.2 .8 Con Constr struct uction ion mat materia erials ls mus mustt be use used d tha thatt are suf suffficiently insoluble and noncontaminating to provide the desired rack life. 4.2.9 Compo Components nents must be placed in such positions that gas from the parts, rack, thieves, masks, and anodes escapes freely and does not become entrapped so as to prevent electroplating on areas that should be electroplated.
removal of the ever-present, tenacious oxide film on the surface of aluminum is what makes electroplating difficult. When using test methods in which a zinc immersion film is applied to the aluminum surface for protection against oxide formation, the article to be plated must enter the chromium-plating solution under live current. 6.1.2 Titanium— Like L ike alu alumin minum um,, tit titani anium um ha hass an eve everrpresent tenacious oxide film that must be removed prior to plating. Practice Practice B481 offers many ways to prepare titanium prior to chrom chromium ium electro electroplatin plating. g. Nickel Alloys— Sever 6.1.3 Nickel Several al dif different ferent activat activation ion metho methods ds are available in Practice B558 Practice B558 for for the preparation of different nickel alloys. The main difficulty with these materials when chromi chr omium um plat plating ing is pol polari arizati zation on of the nic nickel kel allo alloy y sur surfac facee prior to plating which results in deactivation of the material and skip plating. Copper per and Cop Copper per Allo Alloys— ys— Practice 6.1.4 Cop Practice B281 offers many suitable methods for preparing copper and copper alloys prior to chrom chromium ium electro electroplatin plating. g. In genera general, l, only deoxidizing deoxidizing of the copper or copper alloy surface is necessary for chromium electroplating. 6.1.5 Stainless offers many suitab suitable le Stainless Steel— Practice B254 offers activating procedures for the preparation of stainless steel prior to chrom chromium ium electroplating. electroplating. Some stainless steels benefit from a Woods nickel strike prior to chrom chromium ium electro electroplatin plating. g. Polar Polar-ized surfaces surfaces in hig high-n h-nick ickel el stai stainle nless ss stee steels ls can cau cause se ski skip p plating if not properly activated. 6.1.6 Cast Iron— Practice B320 Practice B320 offers offers many suitable procedures for activating cast iron prior to chromium electroplating. In general, anodic etching in the chromium plating solution is not recommen recommended ded.. Due to the high car carbon bon content content in iro iron n castings, casting s, anodic etching leaves a carbo carbon n smut on the surfa surface ce of the metal which results in poor adhesion of the chromium.
4.3 Anodes— Lead Lead anodes containing 4 to 6 % antimony, 4 to 7 % ti tin, n, or 1 % si silv lver er,, or a co comb mbin inat atio ion n th ther ereo eof, f, ar aree satisfactory satisfa ctory.. Chemica Chemicall lead is also satisfactory where hardn hardness ess and rigidity are not important. However, it tends to form great quantities of scale that may fall off on the work and cause pitting or roughness. Lead wire used for small anodes should contain con tain 0.25 % ant antimo imony ny to obt obtain ain the bes bestt rel relatio ationsh nship ip between twee n rig rigidit idity y and duc ductili tility ty in clo close se tole toleran rance ce are areas. as. Lea Leaddsheathed steel, copper, or silver may be used when indicated by requirements for strength or conductivity. Platinum, platinumclad cla d ni niob obiu ium, m, or ev even en ste steel el ro rods ds or wi wire re ma may y be us used ed fo forr inter in terna nall el elect ectro ropl plat atin ing g of sm smal alll ho hole les, s, bu butt th thee lat latter ter wil willl contaminate the bath with iron. If the anode contains little or no lead, the reoxidation of trivalent chromium to the hexavalent state will not take place or will be seriously impaired, which will lead to trivalent buildup in the plating solution and poor results. 4.3.1 Some proprietary proprietary baths may requi require re special anode anodes, s, which should be recommended by the supplier.
6.2 Chrom Chromium ium plating on steel is among the most common combination for engin combination engineering eering purposes. purposes. Uniqu Uniquee activati activation on procedure ced uress for steel exi exist st wit with h chr chromi omium um plat plating ing that mer merit it a separate discussion for successful plating as follows. 6.2.1 Etchin Etching g of the steel before electroplating electroplating is ordinarily ordinarily desirable to obtain satisfactory adhesion of the chromium to the steel. stee l. To red reduce uce the inc increa rease se in rou roughn ghness ess res resulti ulting ng fro from m etch et chin ing, g, th thee et etch chin ing g ti time mess sh shou ould ld be ke kept pt as sh shor ortt as is consist con sistent ent with good adh adhesi esion, on, par particu ticular larly ly in the cas casee of highly finished surfaces. 6.2.2 Anodic Etching in Chromic Acid Solution— The The part to be elec electro tropla plated ted may be ano anodic dically ally etched in a sol soluti ution on of approximately the same concentration of chromic acid as the plating solution (for example, 250 g/L [33 oz/gal]) at approximately the temperature used in plating. There should not be any sulfuric acid present. Enter the tank with the current off and make the part anodic for 10 s to 2 min at a current density of 11 to 32 A/dm 2 [100 to 400 A/ft 2]. Tank voltage is normally 4 to 5 V. There does not have to be rinsing before transfer to the plating tank, but parts should be thoroughly drained to prevent spillage of the etching solution. 6.2.3 Anodic Using ng the Anodic Etch Etching ing in the Pla Plating ting Sol Soluti ution— on— Usi same times and current density described described in 6.2.2 in 6.2.2,, parts can be etched in the plating solution itself. A reversing switch should be pro provid vided ed to mak makee the part ano anodic dic.. Thi Thiss pro proces cesss is muc much h
5. Clea Cleaning ning 5.1 Parts to be electroplated electroplated may be cleaned in accordance with Practices B183 Practices B183,, B242 B242,, B254 B254,, B281 B281,, B320 B320,, B322 B322,, B481 B481,, B558,, or B558 or B630 B630,, or Guide B253 B253.. 5.2 Mecha 5.2 Mechani nica call me meth thod odss of cl clea eani ning ng st stee eell pr prio iorr to electroplating, including abrasive blasting or light grinding, are also suitable. suitable. If par parts ts hav havee bee been n sho shot-p t-peen eened ed to dev develo elop p a compressively stressed surface, it is important to avoid removing that surface by excessive grinding. 6. Deox Deoxidiz idizing ing and Etching 6.1 Prior Prior to chr chromi omium um ele electro ctroplat plating ing,, mos mostt met metals als nee need d special preparation in order to achieve maximum adhesion of the chromium chromium to the substrate substrate.. Dep Depend ending ing on the type and nature of the metal and prior surface preparation steps, various deoxidation and etching methods may be used to activate the substrate prior to chromium electroplating. 6.1.1 Aluminum— Chromium Chromium may be electroplated directly onto ont o mos mostt cas castt and wrought wrought alu alumin minum um mate materia rials ls use used d for engineering purposes. Guide B253 B253 offers offers many useful methods for preparing aluminum prior to chromium electroplating. The 3
B177/B177M − 11 simpler than that in 6.2.2 in 6.2.2 and and requires one less tank, but has the disadvantage of contaminating the bath with iron, copper, and so forth. 6.2.4 Anodic Etching in Sulfuric Acid Solution— A sulfuric acid (H2SO4) solution of 50 to 70 volume % 66 Be H 2SO4 may be use used d for etching. etching. The temp tempera eratur turee sho should uld be kep keptt bel below ow 30°C and preferably below 25°C. The time of treatment is 10 s to 2 min, and the current density 11 to 54 A/dm 2 [100 to 500 A/ft2] at 4 to 6 V. Lead cathodes should be used and the tank constructed constr ucted of a materia material, l, such as lead or vinyl, that is resistant to sulfuric acid. Two difficulties that may be encountered that make this process less attractive than those described in 6.2.2 or 6.2.3 or 6.2.3 are: are: 6.2.4.1 6.2.4 .1 If the rinsin rinsing g following etching is incomplete, the drag-in of sulfuric acid changes the chromic acid to sulfate ratio in the chromium plating bath with deleterious results, and 6.2.4. 6.2 .4.2 2 In han handli dling ng par parts ts tha thatt are dif diffficu icult lt to man manipu ipulate late,, there is a danger of rusting of the surfaces before the part can be introduced into the chromium electroplating bath. 6.2.5 Slight Etching by Acid-Immersion — A slight etching may be obtained by a short dip at room temperature in either 10 to 50 volume % hydrochloric acid (HCl 37 weight %) or 5 to 15 vo volu lume me % su sulf lfur uric ic ac acid id (H2SO4 98 we weig ight ht %) %).. Th This is is normally used on highly finished steel requiring only a thin chromium deposit as its use may result in less adhesion than other procedures and in hydrogen embrittlement of the steel. Drag-over of either solution into the chromium electroplating bath because of poor rinsing will cause contamination problems.
7.2.1 7.2. 1 This This is th thee mo most st co comm mmon on ba bath th an and d wi will ll de depo posi sitt chromium at the approximate rate of 25 µm [0.001 in.] in 80 min at 31 A/dm2 (2.0 A/in2). (Warning— The sulfate anion Warning—The 2– (SO4 ) is added to the bath as sulfuric acid. The calculated amount should be diluted by adding it to deionized water prior to adding it to the bath. Face shield, chemical goggles, rubber glov gl oves es,, an and d ot othe herr sa safe fety ty eq equi uipm pmen entt sh shou ould ld be us used ed wh when en handling sulfuric acid and when making this addition. Consult with appropriate safety manuals or safety personnel, or both, before handling sulfuric acid or chromic acid!) Chromic acid (CrO3) Sulfate (SO42-) Ratio CrO3 to SO4 2Temperature Current density Range
2 5 0 g/ L 2 .5 t o 3 . 1 g /L 80 to 100:1 55°C (range from 40 to 65°C) 31 A/dm2 [2 A/in.2] 25 to 124 A/dm2 [1.6 to 8.0 A/in.2]
NOTE 1—Many factors influence the choice of current densities. With very great agitation, the highest current density shown is possible with a concomitant decrease in the plating time. As the electrochemical efficiency efficiency decreases somewhat with increasing current density and bath temperature, the increase in the plating rate is not linear with the increase in the current density. NOTE 2—Chromium will plate satisfactorily from baths with chromic acid ac id as di dilu lute te as 75 g/ g/L L an and d as co conc ncen entr trate ated d as 50 500 0 g/ g/L. L. The lo lowe werr concentrations give increased efficiency but the throwing power, which is always poor, gets worse. The normal high concentration bath is 400 g/L at the same ratio of chromic acid to sulfate as is used with the common 250-g/L 250-g /L bath. The higher concentration concentration bath gives slightly improved throwing power and a deposit that is less prone to cracking, however softerr in micro softe micro-hard -hardness ness than the comm common on 250-g 250-g/L /L bath.
7.2.2 The fol 7.2.2 follow lowing ing coco-cata catalyz lyzed ed bat bath h giv gives es gre greatly atly improved prov ed ef effficienci iciencies es in compar comparison ison with the standard bath in 7.2.1 under under identi identical cal cond conditions. itions. The additio addition n of fluoride or silicofluoride auxiliary catalysts increase the tendency of the bath to etch steel in unprotected unprotected low-current low-current density areas, and moree mas mor maskin king g may be req requir uired ed tha than n is nec necess essary ary with the standard bath. Analytical control of the silicofluoride is more difficult than the other components, but ion selective methods are satisfactory. This bath will deposit chromium at an appropriate rate of 37.5 µm [0.0015 in.] in 60 min at 31 A/dm 2 [2 A/in2]. (Warning— The silicofl silicofluorid uoridee (some (sometimes times shown as Warning—The fluor flu oros osili ilicat cate) e) an anio ion n ma may y mo most st co conv nven enien iently tly be ad adde ded d as hydrofluorosilicic (fluorosilicic acid), which is commonly sold at a concentration of 31 weight % H 2SiF6, in which case the addition of 1.6 mL/L will give the concentration of 2.0 g/L in the bath. This acid also requires great care in handling. Consult safety references or personnel before using.)
7. Chromium Electropla Electroplating ting 7.1 Unles Unlesss the parts are etched by reverse reverse in the plating bath (6.2.3 6.2.3)), they are introduced into the chromium electroplating bath aft after er all pre prepar parato atory ry ope operat ration ions. s. Any aux auxilia iliary ry ano anodes des integrated with the rack are connected to the anode bus bar. Steel or ferrous parts to be plated are allowed to reach the bath temperature tempera ture and electro electroplatin plating g is then commen commenced. ced. If the parts were etched in the plating solution, plating is initiated when the parts are made cathodic at the end of the etching period using the reversing switch. Most nonferrous metals enter the chromium plating solution under live current and are not placed in the chrom chromium-p ium-plating lating solution for warmin warming g prior to electro electro-plating. 7.2 Electroplating Baths— In In addition to the following two baths, there are various proprietary baths offered that may be satisfa sati sfactor ctory y and sho should uld be ope operat rated ed in acc accord ordanc ancee with the vendor ven dor’s ’s ins instru tructio ctions. ns. Mos Mostt pro propri prietar etary y chr chromi omium um plat plating ing baths are co-catalyzed plating solutions in which an additional catalyst is used in conjunction with the traditional sulfate anion catalyst. These co-catalysts may use organic based or inorganic based compounds to achieve higher plating efficiencies and are ofte of ten n em empl ploy oyed ed wh wher eree hi high gher er ra rates tes of pl plati ating ng an and d be bett tter er throwi thr owing ng and cov coveri ering ng pow power er are nee needed ded.. The mos mostt rec recent ent baths do not use fluoride co-catalysts and do not etch unprotected low current density areas. These baths produce microcracked deposits which may be an advantage in some deposits. There are additives, such as selenium, in the patent-free art which will also produce micro-cracked deposits.
Chromic acid (CrO3) Chromic Sulfate (SO42-) Silicofluoride (SiF62-), see Warning Temperature Current density
2 5 0 g/ L 1 .5 g /L 2 .0 g / L 5 5 °C 31 to 62 A/dm2 [2 to 4 A/in.2]
7.2.3 The fol 7.2.3 follow lowing ing bat bath h pro produc duces es ver very y sof softt (us (usual ually ly less than 650 VHN 25) deposits that are crackfree. The deposits are dull gray in color and can be buffed, if desired. The efficiency is ve very ry hi high gh an and d th thee ch chro romiu mium m ev evid iden entia tiall lly y de depo posit sitss in a different crystal structure than is obtained in other baths. There are man many y mod modifica ificatio tions ns rep report orted ed in the lite literatu rature re and som somee manufacture manuf acturers rs of offer fer prop proprietary rietary baths. (Warning— Literature Warning—Literature refere ref erence ncess sug sugges gestt pre prepar paring ing thi thiss bat bath h by add adding ing sod sodium ium hydroxide to a 4 Mol chromic acid solution. This is a very 4
B177/B177M − 11 dangerous exothermic reaction. The preceding solution should, of course, be handled with all the caution required of standard chromium chrom ium plating baths.) Chromic acid (CrO3) Sodium dichromate (Na2CrO4 2 H2O) Sulfate (SO42-) Temperature range Current density
lower concentration sulfate bath (see 7.2.1 (see 7.2.1). ). Proprietary baths should be evaluated for the tendency towards macrocracking if fatigu fat iguee lif lifee is an imp import ortant ant des design ign con consid sidera eration tion.. For par parts ts loaded in compression or not subject to cyclical applications of stress during operation, or both, this may not be a consideration.1
3 2 5 g/ L 1 7 5 g /L 0 .7 5 g /L 1 5 – 2 5 °C 20 – 90 A/dm2
9. Repair Repair of Chromium Chromium Elec Electro trodepos deposits its on Stee Steell Substrates
7.2.4 7.2. 4 Bla Black ck ch chro romi mium um de depo posi sits ts ar aree pr prod oduc uced ed fr from om th thee following bath. There are also proprietary solutions available. These deposits are frequently used on solar collectors and for applica app licatio tions ns on ste steels els and other allo alloys ys whe where re a mor moree wea wearrresistant coating than black oxide types is desired. In operating these baths, it is essential that no sulfate be introduced into the bath ba th.. Al Alll ba bath thss of th this is ty type pe in inclu clude de ba bari rium um sa salts lts or ot othe herr precipitants for sulfate. As the deposit is nonconductive, the maximum thickness that can be expected is 3 to 5 µm which requires 4 to 8 min. Mild steel anodes are usually employed. Chromic acid (CrO3) Chromic Acetic acid, glacial (CH3COOH) Barium acetate (Ba(CH3COOH)2) Temperature Current density
9.1 A worn chromium chromium electrodepos electrodeposit it may be restored to the original origin al dimens dimension ion by re-elec re-electropl troplating. ating. 9.2 If the par partt is completely completely covered covered in chr chromi omium um in the areas originally electroplated, it may be prepared for electroplating in accordance with Practice B630 Practice B630.. 9.3 If steel shows through through or if the anodi anodicc treatment exposes exposes steel, the chromium coating must be completely removed prior to re-electroplating. Stripping the chromium may be done by anod an odic ic tr trea eatme tment nt at 5 to 8 A/dm2 [75 A/ft2] in a so solu luti tion on containing 40 to 60 g/L of sodium hydroxide or in a solution contain con taining ing 40 to 60 g/L sodium sodium car carbon bonate. ate. Eith Either er sol solutio ution n should be kept below 25°C during operation using cooling, if necessary. There are also proprietary solutions available which should be operated according to the supplier’s instructions.
2 5 0 g /L 20 m L/ L 2 0 g /L 2 0 – 4 0 °C 5 – 15 A/dm2
8. Treatme reatments nts of Chromium Coatings Hydrogen Embrittlem Embrittlement— ent— Hydrog 8.1 Hydrogen Hydrogen en evo evolved lved dur during ing chromium plating is apt to embrittle steel, and the potential for embritt emb rittleme lement nt incr increase easess wit with h the hig higher her str streng ength th (ha (harde rder) r) steels ste els.. Bak Bakin ing g ap appr prop opria riate te fo forr th thee ten tensi sile le st stre reng ngth th of th thee electro elec tropla plated ted par partt mus mustt be per perfor formed med to red reduce uce the risk of hydrogen hydro gen embrit embrittlement tlement.. Guide Guide B850 lists bakes appropriate for the tensile strength of the electroplated part and should be consult con sulted ed for pos post-el t-electr ectropl oplatin ating g bak baking ing pro proced cedure uress and classes. In all cases, the duration of the bake shall commence from the time at which the whole part attains the specified temperature. The bake should be performed as soon as possible after the parts are removed from the plating bath, rinsed, and dried in order to reduce the risk of hydrogen embrittlement. Consult Guide B850 for maximum length of time permitted between plating and baking operations.
10. Test Methods 10.1 Guide B697 B697,, with Test Methods B602 Methods B602 and and B762 B762,, will be helpful in choosing statistically appropriate sample sizes for the following test methods. 10.2 Thickness— The The thickness of the chromium deposit is usually not determined directly, the dimension of the finished part being measured instead. When direct measurement of the thic th ickn knes esss of th thee co coat atin ing g is de desi sire red d an and d th thee pa part rt ca can n be sacrificed, it should be done in accordance with Test Method B487.. If a nondestructive method is required, magnetic inducB487 tion tio n me meth thod odss in ac acco cord rdan ance ce wi with th Tes estt Me Meth thod od B499 are suitable for chromium over magnetic substrates. Test methods in accordance with Test Methods B499 can measure coating thicknesses from 2.5 µm to 12 mm [0.1 mil to 0.5 in.]. Test Method B244 Method B244 may may be used accurately for chromium up to 500 µm [0. [0.020 020 in.] over alu alumin minum um or cop copper per alloys alloys but not for titanium. For deposits up to 50 µm [2 mils], Test Method B504 Method B504 may be used and does not destroy the part, but does remove the chromium electrodeposit on the area tested, which may necessitate replating. X-Ray fluorescence may be used to measure very thin chromium deposits of 1 to 20 µm [3 µin. to 0.8 mils] in accordance with Test Method B568 Method B568.. Other test method methodss may be used by agreement between the purchaser and the seller.
NOTE 3—It is suggested that the selection of an appropriate bake be discussed with the purchaser to ensure that the bake selected does not cause distortion in the part or adversely affect its mechanical properties. NOTE 4—The effectiveness of hydrogen embrittlement relief baking of chromium-p chrom ium-plated lated high-strength high-strength steels can be tested in accord accordance ance with Test Method F519 Method F519..
Mechanical Finish Finishing— ing— Chromium 8.2 Mechanical Chromium electro electrodepos deposits its are commonly finished to the required final dimension by grinding, grinding and honing, or lapping. If grinding is very aggressive, remo re movi ving ng a la larg rgee am amou ount nt of me metal tal pe perr gr grin indi ding ng pa pass ss an and d generating genera ting high localized temperatures, temperatures, the chrom chromium ium is apt to develop a network of macrocracks visible to the naked eye. This condition will greatly reduce the fatigue life of the part and should be avoided. Compressively stressing the substrate surface prior to plating by shot peening (see Specification B851 or MIL-S-13165C, or both) or other means will help prevent any diminution of the fatigue life. Chromium deposited from the higher concentration sulfate catalyzed baths are less prone to macrocracking during grinding than those deposited under similar conditions from a cocatalyzed bath (see 7.2.2 7.2.2)) or the
10.3 Hardness— Hardness Hardness will vary with bath composition and the conditions used for electrodeposition. Hardness should be measured in accordance with Test Method B578 B578 on on a panel plate pl ated d co conc ncur urre rent ntly ly wi with th th thee pa part rt un unles lesss th thee pa part rt can be sacrificed. 10.4 Adhesion— Adhesion Adhesion should be measured using Practice B571 on tice B571 on a panel plated concurrently with the part and on the same material as the part. 11. Keywords 11.1 11 .1 chrom chromium ium electroplating electroplating 5
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