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"It Worked For 20 Years, Now The Latest One Lasted 2
Years, What Gives?"
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The Case Of The Vanishing Molybdenum.
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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. |
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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. |
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Remedy |
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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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