Conventional diffusional heat treatments are usually performed when an increase on the surface hardness of a certain component is desired. However, some alloys, such as duplex stainless steels and nickel alloys cannot be submitted to the temperature ranges that these treatments require due to the precipitation of deleterious phases. Besides that, since these alloys are used in highly corrosive environments, the effect of those treatments on corrosion resistance is a point that deserves special attention.
This paper presents a study about the effect of two different diffusional heat treatments on the corrosion behavior of two nickel alloys (UNS N07718 and UNS N09925). The first one (HT#1) can be classified as a carburizing process, and the second (HT#2) as a nitriding process. Treated and untreated samples were analyzed through a corrosion test called critical pitting temperature (CPT) that was determined through an electrochemical method, according to ASTM G150, and by a chemical one, per ASTM G48. The results showed that the heat treatments presented opposite effects on the substrates. HT#1 improved the corrosion resistance of both substrates, although 925 HT#1 presented a lower CPT value by ASTM G48, which can be explained due to the test method aggressiveness, and HT#2 reduced their CPT. In all cases where pits were observed, they were macroscopic.