8/8/2019 AT0024--Chloride Stress Corrosion Tech Brief

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Chloride Stress Corrosion CrackingResistance of High PerformanceCopper Alloys

by John O. RatkaBrush Wellman Inc.

Stress Corrosion Cracking (SCC) is an

environmentally assisted failure caused byexposure to a corrodant while under a sustainedtensile stress. SCC is most often rapid,unpredictable and catastrophic. Failure can occurin as little as a few hours or take years to happenand is comonly observed in the absence of otherforms of corrosion, such as general or crevicecorrosion.

Most alloys are susceptible to SCC in one or moreenvironments requiring careful consideration ofalloy type in component design. In oxygenated

aqueous chloride environments austenitic stainlesssteels and many nickel based alloys are known toperform poorly. These alloys demonstrate a rapiddecrease in failure time with increasing appliedstress. Use of these alloys often requires additionalprocessing, such as shot peening, cathodicprotection or the application of protective coatingsto prolong the structure life. However, theadditional processing requirements add significantlyto the cost of use.

Numerous service conditions exist where high

chloride conditions are present. Aqueous chlorideenvironments are common in the Oil and Gasindustry in both the downhole and productionsituations. Highly concentrated hot brine orpotassium chloride are common in explorationdrilling requiring the use of chloride SCC resistantmaterials such as high copper alloys. Seawaterconditions are another area where copper berylliumalloys, for example, have been exceptionallysuccessful for undersea electronic and fiber optichousings.

A method of commonly evaluating materialsusceptibility to Chloride SCC is ASTM StandardPractice G36, Performing Stress-CorrosionCracking Tests in a Boiling Magnesium ChlorideSolution. This accelerated test methodincorporates C-ring specimens of various alloys ofsteel, nickel and copper to determine the failuretime as a function of applied stress. Thespecimens evaluated were loaded to an applied

stress of 100% of the yield strength and immersedinto the solution as described by the standardpractice. If the specimens failed in less than 1000hr, additional specimens were loaded to 50% of theyield strength and tested in solution. Copperberyllium alloy C17200 was tested at three levels ofstrength to determine the effect of aging onresistance to chloride SCC.

The results shown in the graph indicate that thestainless steel alloys are highly susceptible tochloride SCC, as are some of the nickel-basedalloys. These alloys fail by cracking in times asshort as several hours, even at a fraction of theiryield strength. The data also show that severalhigh performance copper-based alloys demonstrateSCC resistance at strength levels equal to, orgreater, than several SCC resistant nickel alloys.Copper beryllium C17200, for example, providesthe greatest strength and SCC resistance of thealloys evaluated. The excellent corrosionresistance is independent of strength, shown at0.2% yield strengths ranging from 100 ksi to 150ksi. A lower strength, but higher conductivitycopper beryllium alloy, C17500, is also shown to beresistant to chloride SCC. A new commerciallyavailable spinodally hardened Cu-Ni-Sn alloy,Brush ToughMet 3 AT, offers SCC resistance at ayield strength of 110 ksi.

When design considerations require a chlorideSCC resistant alloy, other material properties andattributes should also be considered. For example,

8/8/2019 AT0024--Chloride Stress Corrosion Tech Brief

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AT0024/0800

instrumentation for directional sensing duringdownhole oil and gas exploration may require highstrength and non-magnetic behavior. Otherdownhole drilling situations require bending of adrill string over a tight radius defining the need for alow modulus alloy which reduces generated stresslevels. Wear and friction behavior may be

necessary for bushings and wear surfaces exposedto SCC inducing environments. These designneeds eliminate the high nickel alloys from theapplication because of their ferromagnetic

behavior, high elastic modulus and propensity forgalling and seizing. High strength copper alloys,such as the copper beryllium and spinodal alloys,are the engineering solution in many applicationswhere the material demands are high.

If you require additional information, or technical

assistance, please contact Brush WellmansTechnical Service Department at (800) 373-4205.

Chloride SCC

Time to Failure (hr)

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AppliedStress(ksi)

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C17200

S31600S17400S20910N09925SCF-19C72900C17510N05500N07031

C17200 - Brush Wellman Alloy 25 AT; Tube, PA 150 ksi YS; Forged UA, 110 and 100 ksi YSS31600 - AISI 316 SS A, Tube, 40 ksiS17400 - 17-4 PH, Bar, 127 ksi YSS20910 - Nitronic 50, 103 ksi YSN09925 - Incoloy 925, 106 ksi YSSCF 19 - SCF-19 (Carpenter Technology), 120 ksi YSC72900 - Brush Wellman Toughmet 3 AT, 110 ksi YSC17510 - Brush Wellman Alloy 3 AT Tube, 80 ksi YSN05500 - Monel K500 (Huntington Alloys, Inc.), 112 ksi YSN07031 - Pyromet 31 (Carpenter Technology), 106 ksi YS