A study was made on the corrosion behavior of various stainless steels and low alloy steels in three types of ammonia converters. Test periods were 230, 1, 200 and 1, 986 days for austenitic stainless steels.
(1) Corrosion rate of stainless steels was ranged from 0.06 to 0.15mm/yr, and de-carburization occurred in nitriding layer of stainless steels. When stabilizing elements such as titanium and niobium were sufficiently added to steel, the de-carburization in nitriding layer was prevented. The thickness of nitriding layer was decreased with increasing nickel content. The metallographic structures were determined by X-ray diffraction method. These results showed that the layer consisted of ferrite, austenite and chromium nitride, whereas the core was austenite only.
(2) The nitriding layer was formed in the low alloy steels, i.e. 21/4 Cr-Mo steel, 5 Cr-Mo steel, 9 Cr-Mo steel and Cr-Mo steels stabilized with titanium, niobium and vanadium. The Cr-Mo steel, 31/2 Ni steel and Ni-Cr-Cu steel were nitrided from surface to centre, and also microcracks were observed. Carbon was scarecely detected in nitriding layer of 5 Cr-Mo steel and 9 Cr-Mo steel, and cracks were not observed. On the contrary, both the decarburization and microcracks were not observed in the nitriding layer of titanium and niobium-bearing Cr-Mo steels.

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The influence of oxidizing agents on the pitting dissolution of an austenitic stainless steel, SUS33, in 0.5N sodium chloride solution was studied electrochemically using an artificial pit method.
The dissolution behavior of the pit anode was divided into following three states. (1) Active state of dissolution with a very high rate. (2) Intermediate state between active and passive with a relatively higher rate than that of the ordinary passive state dissolution. (3) Inactive state corresponding to the passive state for the bulk cathode surface.
Oxidizing anions can make the active pit anode change to the intermediate state described above as the anion concentration is more than a critical concentration against that of the chloride ion. This condition can alter the inactive state to disappear the pitting dissolution.
For the oxidizing anions studied, the inhibitive efficiency for the pitting dissolution estimated from the critical concentration decreases in the order: nitrate>nitrite>permanganate>dichromate>chromate> perchlorate.
It is assumed that the area of active dissolution on the surface of pit anode decreases in the intermediate state through the adsorption of oxidizing anions on a part of active point, therefore the appearent current density of pit anode shows a intermediate value between those of ordinary active and passive states.
Oxidizing cations, i.e. ferric and cupric ions, have no effect to change the pit anode to the inactive state.

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