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"It Worked For 20 Years, Now The Latest One Lasted 2 Years, What Gives?"  

The Case Of The Vanishing Molybdenum.

 

Molybdenum is a key component in corrosion resistant alloys in many processes - particularly those processes that are prone to pitting, crevice corrosion, or stress corrosion cracking.  In Type 316 stainless steel, the amount of molybdenum in the alloy is required by ASTM to be in the range of 2% to 3%.  Thirty years ago, when casting practices at the foundries were not as closely controlled as today, the targeted amount of molybdenum was 2.5% and, many times, components were produced with molybdenum contents on the high-side of the range.

Within the past twenty years, cost conscious observers noticed that their foundries routinely achieved the 2.5% target because the alloy manufacturers can more closely control the ladle composition.  Consequently, Type 316 is now being routinely produced with molybdenum content on the low end in the 2.05% to 2.10% range. As a result, the corrosion resistance of the alloy, in some applications, has suffered.

Remedy

If your process has successfully used Type 316 stainless steel in the past, but the equipment is being replaced after many years of satisfactory service, consider upgrading to an alloy such as Type 317 which contains 3 to 4% molybdenum.

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