How to avoid fai of stain ess stee Preventing stress-corrosion cracking, These alloys are easy to fabricate and offer proven 1 urrformance. They are the standard allov selections. pitting. and crevice corrosion is the 1 ' The new ferritic'and duplex stainless s;eels will not be -

discussed. These were dealt with recently in this depart- key to success. Knowing when to use ment by Kedmond and Miska [ I ] . a stainless, and when not to, is crucial.

C. M. Schillmoller, Sctl rlltr~oller Assoclutes, and Heinz J. Althoff, khe~nlgte Deuhche !\Zetallwrrke AG

For stainless steels, almost 60% of equipment fail- ures in the chemical-process industries (CPI) are d u e to stress-corrosion cracking (scc:), or localized attack in the form of pitting or crevice corrosion.

Operating parameters and environmental factors greatly affcct alloy performance, especially pH, tempera- ture, and chloride and oxygen levels. Failures of stain- lesses can be avoided by understanding their limits in lipht of the above variables

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This discussion will Pocus on stainless steels, but will include sornt other alloys used for chloride service- high-molvbdenum nickel alloys. It is not possible to review all of the spec~allv stainlesses, chronle and N i - 0 - Mo alloys. Coverage will bc limited to several of the austenitics. Included are Types 304L, 31GL and Y17L.

T h e table lists some of the high-technology stainless steels and other alloys used in CPX plants. The alloys listed, as well as some others, will he looked at in this article in terms O ~ S C C , and pitting and crevice corrosion.

SCC: Environmental factors Generally, for stainless steels to suffer S(:C, chloride

concentrations must be 30 ppm or higher. However, when chlorides are present in trace amounts, high enough concentrations may develop locally that will cause cracking.. This happens, for example, via chloride enrichment on hot surfaces, such as at vapor-liquid interfaces under deposits and in crevices. Proper design can often eliminate places where chlorides can concen- trate. Oxygen is necessary for scc to occur, and is usually present in small amounts.

Most failures occur above 170°F (75"C), and 110°F (.50"C) is generally considered the threshold below which SCC is not a problem.

Tenslie stresses are necessary for the propagation of

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Stainless steels a n d high-nickel al loys for use in t h e chemica l process industries

Generic name 316L 317CM 904 904hMo 28 20 825 825 hMo G C-276 625 Stainless steels High-nickel alloys

Cronifer 1925 LC, Cronifer Nicrofer Nicrofer Nicrofer Nicrofer Nicrofer 904L, 1925 hMo. Nicrofer 3620 Cb, 4221, 4520 IiMo, 5716 hMoW, 6020 hMo

Tradenames and Cronifer Cronifer 2RK65, AL-EX, 3127, Carpenter lncoloy Nicrofer Hastelloy Hastelloy lnconel comparable products 1812 LC 1713 LCN JS700 254SMo Sanicro 28 20Cb-3 825 4221 hMo G ------ -- C-276 625

Chemical composition (nominal analysis, %) C max. 0.03 0.03 0.02 0.02 0.02 0.03 0.025 0.025 0.015 0.015 0.10 Cr 17.5 18.0 20.5 20.5 27.0 20.0 21.0 21.0 22.0 16.0 21.5 N i 13.5 13.0 25.0 25.0 31.0 37.5 42.0 42.0 45.0 57.0 61.0 M o 2.75 4.5 4.7 6.0 3.5 2.5 3.0 6.0 6.4 16.0 9.0 Cu - - 1.5 1.5 1.0 3.5 2.5 2.5 2.0 - - N - 0.14 - 0.12 - - - - - - - CbiTa - - - - 2.0 - 3.65 - 0.3 - -

Others - - - - - Cb T i Al, T i , Co Al, Ti Al, T i -

Wirksumme 26 32 36 40 38 28 31 41 4 3 N I A 51 Tradenarnes: N icro fer and Cronifer-VDM Germany; Incoloy and Inconei-International Nickei Co.; Cartech-Carpenter Technology Corp.; tiastelloy-Cabot Corp. 9 0 4 L is produced by Uddeholm; 254SMo b y Avesta; 2RK65 and Sanrico 28 by Sandvik; A L 4 X b y Al legheny Ludlurn; and JS700 b y Jessop Steel.