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Corrosion of unalloyed steels in different alkaline solutions at high temperatures and under high pressures W.M.M. Huijbregts Kema Scientific & Technical Reports 1 (1): 1-9. ISSN 0167-8590; ISBN 90-353-0005-X. (paper 23) Abstract The non-volatile chemicals NaOH, KOH, LiOH and Na3P04-Na2HP04 are used as acidchloride corrosion inhibitors in evaporators, at high temperatures and under high pressures. The alkaline corrosivities of these chemicals were determined by autoclave studies. There is a critical hydroxide concentration for NaOH, KOH and LiOH above which the corrosion rates increase dramatically. LiOH appears to have the lowest critical concentration (0.5 mol). The corrosion rate in high concentrations of LiOH is five times faster than in NaOH. Many heats were performed on a number of steel classes in highly concentrated hydroxide solutions. There appears to be a wide variation in the corrosion rates. Some results
Fig. 1. Weight losses after a 4-day exposure to NaOH under non-deaerated conditions at 250 °C (experiment 1).
Figure 3. Weight losses after a 4-day exposure under non-deaerated conditions at 310 °C (LiOH and NaOH)(experiment 2b).
Conclusions (1) Above a critical hydroxide concentration the corrosion rate increases rapidly with increasing hydroxide concentrations. (2) The critical concentration of LiOH amounts to about 0.5 molar and that of NaOH 2-3 molar. KOH shows a critical concentration of 1:5 molar. (3)The critical concentration for alkaline corrosion was only determined for a few heats of the 5 steels tested. It is not clear whether this critical concentration is also controlled by the chemical composition of the steel within the broad range of the various steel specifications. (4)The critical alkaline concentration at 250 °C does not differ from that at 310 °C. (5) The corrosion rate in highly concentrated (4 and 10 molar) LiOH is five times higher than that in NaOH. (6) Under the isothermal autoclave test conditions in 4 mol NaOH a so-called Potter and Mann oxide layer formed, leading to a parabolic corrosion rate. In 4 mol LiOH, however, a porous columnar oxide structure was formed which results in linear corrosion. (7)The corrosion rate in 10 mol NaOH for carbon steels varied considerably for the 83 different heats. Carbon steel showed the highest corrosion rates. Steels 15Mo3, 14Mn4 and 13CrMo44 had about the same corrosion rates but were only slightly better than carbon steel. Steel l0CrMo910 had the lowest corrosion rate. (8) In the case of a fouled evaporator and the consequent danger of steam blanketing, NaOH and KOH are considered to be better acid-chloride corrosion inhibitors than LiOH. LiOH will give a high alkaline corrosion risk because of the low critical hydroxide concentration. Phosphate is a good inhibitor but due to its low solubility it gives a high fouling effect.
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Oxide layers (x 100) formed on St35.8, number 2, after a 31-day exposure to NaOH (A) and LiOH (B) in 10-molar solutions under non-deaerated conditions at 310 °C (experiment 6)
