Liquid Metal Embrittlement

Form 10001 (7/96)

Metallurgy Advisory

METALLURGY ADVISORY Liquid Metal Embrittlement

MA-E001 J. Sievert January 29, 1998

Problem, Risk and possible Resolutions REV: 0

Revision No. Date By Checked By Approved By

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0 29 Jan 98 J. Sievert n /a n /a

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Form 10001 (7/96)


Metallurgy Advisory MA-E001 J. Sievert January 29, 1998


PAGE 2 OF 5 TABLE OF CONTENTS

Section 1. 2. 3. 4. 5. 6

Title Page No. GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Content Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Liquid Metal Embrittlement, General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Detection of Zinc on Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Removal of zinc from contaminated stainless steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Internet source on Liquid Metal Embrittlement Attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1. General 1.1 The embrittling effect on stainless steels in contact with liquid zinc has been the subject of a number of experimental studies. It is important to view historical occurrences of liquid metal embrittlement and the potent ial f or LME in any contamination event with a degree of perspective. For example, failure consequence of LME might be considered severe when the contamination involves high temperature and high pressure piping or equipment. W hereas, where the contamination might be contained, as within a fired heater firebox, or with systems where no elevated temperatures are anticipated, then the consequences of failure might be viewed as not severe; or at most damaging. 1.2 The following provides sufficient information to allow detection and removal for those instances wherein contamination exposures and the risk for occurrence of LME are high or that removal might be considered necessary. 2.0 The following is covered:

2.1

!

Liquid Metal Embrittlement (LME) - General

!

Detection of zinc on surfaces

!

Removal of zinc from contaminated stainless steels

!

Internet source on Liquid Metal Attack

For attack to occur, it is generally essential that the liquid metal, such as zinc, first become liquid, then in an unoxidized state the liquid must contact the high alloy surface. Further, the alloy surface must be at an elevated temperature on the order of 1112 F (600 C) or over. E

E

2.2

It is pointed out that zinc removal techniques are not always fullproof; and, where required, the removal of all contamination can prove difficult. It is often prudent to ensure that there is or that there is not substantial failure risk in the event that zinc cannot be converted to innocuous zinc oxide prior to removal attempts.

3.0

Liquid Metal Embrittlement (LME) - General

3.1

Zinc and other metals such as mercury, cadmium and lead can rapidly destroy metallic surfaces and produce a significant and rapid embrittlement of alloys such as stainless steels and in some instances for high strength steels. It is pressure boundary high alloys which are generally of concern and the circumstances leading to rapid deterioration include welding galvanized steels onto stainless steels as well as paint oversprays, marking pens and fire conditions within a refinery or petroc hemical process. In addition, there have been cases involving failures due to fabricati ons which brazed thermocouples onto high alloy steels. While prohibitions or precautions to avoid these situations are important, instances of contamination remain to occur. The following addresses some of the experimental and historical inf ormation available.

3.2

In a 1984 API Refining Department Midyear Proceeding, a paper was included entitled “embrittlement of SS welds by contamination with zinc rich paint”. This discussed two “types” of embrittlement processes and related that no zinc embrittlement has been observed at

Form 10001 (7/96)




PAGE 3 OF 5

temperatu res below 1380 F (748 C) even though the melting point of zinc is 787 F (419 C). The types of processes limiting these temperatures were related probably as f ollows: E

3.3 Type 1

E

E

E

“Slow process controlled by rate of zinc diffusion along austenitic grain boundaries. Zinc combines with nickel, leaving nickel depleted zones and results in fcc (face centered cubic) austenite transformation to bcc (body centered cubic) ferrite.”

3.4 Type 2 “Rapid pr opagation from Type 1 at higher temperatures (1650 F/898 C) with crack lengths over 20 times the diameter of the init iating zinc droplet diameter. E

E

3.5 Based on other information and pri or experimental studies, the following have been established: !

The presence of molten zinc is necessary for embrittlement to occur.

!

The lowest reported temperature for zinc embrittlement in laboratory studies has been 1112 F (600 C). [Others have reported the required minimum temperature to be 1382 F (750 C)]. Very high stresses (greater than yield stress) and plastic strain of the material is required for embrittlement to occur at this lower bound temperature. E

E

!

E

The vast majority of f ield evidence of embrittlement has been associated with equipment fir es and/or welding operations where the metal temperatures experienced were significantly higher than 1112 F (600 C). E

!

E

The oxidation rate of zinc metal, if present on the surface of a stainless steel component at temperatures above 500 F(257 C) is very high. Accordingly any zinc on the surface and exposed to air will be converted to an innocuous oxide in a very short time. The melting point of the zinc oxide is above 3200 F (1758 C). E

E

E

3.5

E

E

Owing to the lowest temperature for occurrence at 1112 F (600 C) and the high oxidation rate above 500 F(257 C), a substantial “window” of converting unoxidized zinc into an innocuous oxide is available. E

E

E

E

3.6

Where this air environment temperature “window” is unavailable, then other assessment and removal methods can be considered.

4.0

Detection of zinc on surfaces

4.1

A sensitive field chemical test for detecting the presence of zinc on piping (or other metallic surfaces) consists of a solution of dithizone and 10% sodium hydroxide. This however detects both innocuous zinc oxide. The solution will cause the contaminated areas to show a “pink” color if zinc (or zinc oxide) is present. The method is highly eff ective for determining presence of zinc which has been smeared on surfaces, but is ineffective in the case of ferritic steel splatter (as might occur from arc cutting galvanized surfaces).

4.2

Energy Dispersive analysis (EDX) is a sensitive laboratory technique for detecting the presence of trace quantities of zinc existing on surfaces of metals.

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5.0

Removal of zinc from contaminated stainless steels

5.1

Removal methods such as chemical and mechanical cleaning techniques or combinations thereof can be applied. However, the eff ectiveness of the method varies depending on the surface smoothness, the existence of discontinuities or laps along with the thoroughness of the cleaning. The results of prior experience laboratory testing (by other than M.W. Kellogg) to remove zinc contamination on stainless steel piping surfaces was as follows:

5.1.1 50% Nitric acid swabbing at ambient temperature -

Satisfactory for removing metallic zinc smeared from galvanized sections on smooth surfaces as well as on surf aces containing discontinuities/laps and score marks.

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Unsatisfactory for removing adherent metallic deposits/weld shot nodules (as contact times are too long).

5.1.2 Grinding with a rotary flapper wheel containing (80 grit) aluminum oxide grit paper. -

Satisfactory for removing metallic zinc smeared on smooth surfaces from galvanized surfaces from galvanized sections.

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Satisfactory for removing metallic deposits and weld shot nodules.

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Unsatisfactory for removing discontinuities/laps/score marks.

all

traces

of

zinc

from

surfaces

containing

5.1.3 Wire Brushing with a motor driven rotary stainless steel wire bristles (0.014-inch diameter). -

Satisfactory for removing metallic zinc smeared on smooth surfaces f rom galvanized sections.

-


-

Unsatisfactory for removing discontinuities/laps/score marks.

all

traces

of

zinc

from

surfaces

containing

5.1.4. Grit Blasting with 80 grit aluminum oxide. -

Satisfactory for removing metallic zinc smeared on smooth surfaces from galvanized surfaces from galvanized sections.

-


-

Satisfactory for removing all traces of zinc from surfaces containing discontinuities/laps/score marks.

Form 10001 (7/96)

METALLURGY ADVISORY Liquid Metal Embrittlement Problem, Risk and possible Resolutions REV: 0


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6.0 Internet source on Liquid Metal Embrittlement Attack 6.1

Liquid Metal Attack ---- Description (source http://www.intercor.com/liqmetal.htm): Corrosive degradation of metals in the presence of certain liquid metals s uch as mercury, zinc, lead, cadmium. Examples of liquid metal attack inc lude: chemical dissolution. metal-to-metal alloying (i.e. amalgamation). embrittlement and cracking. Prevention or Remedial Ac tion •selection of compatible materials. •removal of liquid metal from environment. •application of resistant surf ace coating or treatment to act as a barrier between metal and environment. •chemical dissolution and amalgamation - see test methods for general corrosion and pitting. •liquid metal embrittlement - see test methods for scc . Standard Test Methods •ASTM G129 - slow strain rate test for determination of environmentally assisted cracking. •ASTM G-30 - practice for making and using U-bend SCC test specimens. •ASTM G-38 - practice for making and using C-ring SCC test specimens. •ASTM G-39 - practice f or preparation and use of bent-beam SCC test specimens. Evaluation for Liquid Metal Embrittlement (LME) The evaluation of LME usually requires chemical or mechanical techniques to overcome the incubation period for cracking. In much the same way that a localized corrosion event is needed to initiate SCC, local chemical attack is us ually a precursor for LME. Dynamically applied loads as in the slow strain rate test can be used to break normally protective surface films to allow intimate contact of the material sur face and the liquid metal. Chemical agents can also be used t o remove or breach this surface films and initiate localized attack so that the inherent susceptibility of the material can be determined. In some cases, surface treatments may be utilized to enhance resistance to LME. However, this should be conducted with extreme caution since damage to this surface layer may induce cracking.

end http://www.intercor.com/liqmetal.htm

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Liquid Metal Embrittlement

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