Corrosion resistance of Stainless Steel in Methane Sulfonic Acid 70. Influence of MSA Manufacturing route

摘要

Methane Sulfonic Acid (MSA) is a very interesting acid that is increasingly used in many fields like: cleaning, descaling , electroplating but also as catalyst for esterification reactions in the production of phtalate plasticizers, acrylates like multifunctional acrylates, acetates. This is due to a set of very interesting properties among them are: strong acidity, high solubility of calcium salts, biodegradability and, for esterification processes, high selectivity of the reaction with less by-products. MSA is most often delivered, transported and used as aqueous solution (70 wt %). The only drawback of MSA is its high corrosivity, for sure for carbon steel but also for standard stainless steels (SS), which are AISI 304, AISI 316 and their low carbon or Ti stabilized versions. On the contrary to a recent belief, corrosivity of MSA is intrinsic to this acid and is absolutely not dependent on the manufacturing process of MSA. A few authors stated that MSA issued from a chloro oxidation process is corrosive because of the presence of chlorides and that on the contrary, MSA coming from the air oxidation process is not corrosive to stainless steels. However, making corrosion tests of SS in 70% MSA is difficult and it is very easy to have good behavior of SS and to conclude that SS are fully resistant in this acid. When we immerse coupons in 70% MSA, sometimes corrosion starts immediately and sometimes the alloy is passive from the beginning. Immersion tests as well as electrochemical experiments were conducted on Aisi 316L stainless steel (1.4404 according to EN 10028-7). The results show the random behavior of the stainless steel in MSA and allows to explain it by the metastability of the passivity. One of the main conclusion is that using standard SS in 70% MSA is quite dangerous because depassivation of the alloy can arise for any reason, even if the alloy is fully passive at the origin. Moreover, that this is true whatever the manufacturing route of the acid. The present article describes also a newly developed 70% MSA grade (named MSA LC for Low Corrosion) that is the only able to assure a long term fully passive behavior of standard stainless steels, even for high temperature applications like esterification.