Corrosion of Stainless Alloys in Potash Brines
Blackmore, Craig Edward
The temperatures and concentrated chloride environments involved in potash milling provide conditions for high susceptibility to localized corrosion. This must be countered with careful material selection that provides adequate protection at affordable cost. Indexes have been formulated to take into account the effect of the elemental composition of a passive alloy on the resistance of the alloy to localized corrosion. A very comprehensive index, the Localized Corrosion Resistance Index (LCRI), is studied by this thesis and is evaluated for its accuracy in ranking stainless alloys. The LCRI, Cr%+3.3(Mo%)-0.33(Ni%)+16(N%), takes advantage of the readily available weight percentages of chromium, molybdenum, nickel, and nitrogen. This thesis examined the theories behind pitting and crevice corrosion and the effect these elements have on the passive layers protecting the surface of a stainless alloy. Electrochemical experiments were performed on selected alloys in laboratory conditions, a pilot plant flow loop, and in potash mill conditions. The data obtained for this thesis supported the use of the LCRI as a ranking index for austenitic and duplex stainless steels. Ferritic and cast austenitic steels had less resistance to localized corrosion than the LCRI indicated. Totally different indexes or radical correction factors would be required to allow comparison of all types of alloys. A separate and extensive study could eventually achieve this. Temperature effects were examined to determine if the ability to accurately rank alloys with the LCRI could be compromised. The experiments tracked the pitting resistance of the alloys over a temperature range of 20-100°C, covering conditions found in potash mills. The results found the LCRI reliable over the whole range of temperatures. Chromium, molybdenum, nickel, and nitrogen factors in the LCRI were studied, each in tum, to evaluate their worth in predicting the localized corrosion behavior of passive alloys. The data supported the theories regarding the effect of each element on the passive layer protecting the surface of the alloys. A new index was formulated to better fit the experimental data. The formula Cr%+4.1 (Mo%)0.14(Ni%)+6(N%) indicates molybdenum had a more pronounced effect on corrosion resistance while nitrogen's beneficial effect and nickel's detrimental effect were less pronounced. This formula, denoted LCRIll, offers better accuracy than the more conventional LCRI.