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  • Narasi Sridhar

Methanol corrosion

We all know that water is corrosive. In fact, in many industries, the assumption is that without water there is no corrosion and efforts are made to dry (dehydrate) equipment using a number of methods. However, the very act of dehydration can bring on other risks of local attack and stress corrosion cracking. Methanol is a dehydrating agent and is often thought to be innocuous from a corrosion perspective. Carbon steel is used to store methanol and people since the 1980's knew that carbon steel can crack in the presence of methanol. There are well known precautions, like adding a small amount of water, reducing oxygen, and heat treatment to relieve stresses, to avoid this problem. However, it was though that stainless steels were immune to cracking in methanol. In fact the Methanol Institute identifies stainless steels as not affected by methanol. But, when a certain stainless steel is exposed to methanol containing as little as 6 ppm chloride and temperatures of about 80 C can crack. This phenomenon has been presented in a recent paper: Sridhar, N. and L. Cao, Stress Corrosion Cracking of 13Cr-5Ni-2Mo Martensitic Stainless Steel in Methanol. Corrosion, 2022. 78(8): p. 740-750.

Stainless steels and other corrosion resistant alloys can also pit in the presence of methanol. The pitting can occur even at room temperature, depending on the alloy and chloride concentration. However, when sufficient water is present, their corrosion resistance is high. This is because water plays an important role in restoring the protective oxide film on these alloys. The role of water has been examined in a recent paper: Gonzalez, A.H.-., N. Sridhar, and R. Thodla, Localized Corrosion of Chromium-containing Conventional and High Entropy Alloys in Methanol and Aqueous Chloride Environments. Corrosion 2022.

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