Corrosion Resistance in Various Environments

Atmospheric Environments

Nickel and nickel alloys demonstrate very good resistance to corrosion in atmospheric environments, although after extended periods of exposure many alloys will develop a thin, adherent film, especially in industrial environments. Even so, nickel and its alloys are generally suitable for use in atmospheric environments, due to their strong corrosion resistance. Table 35 gives corrosion data on some nickel-based alloys after exposure to the atmosphere. 

Water Environments

Fresh water environments do not pose much of a threat to nickel and nickel alloys, since they generally exhibit a good corrosion resistance in these environments. Therefore, they are suitable for applications that require the exposure to or handling of fresh water environments.

The resistance of nickel and nickel-based alloys to corrosion in seawater is dependent on factors such as the velocity of flow of seawater. Some alloys may exhibit good corrosion resistance to flowing seawater, for example, but are susceptible to corrosion in stagnant or low-flow seawater. Table 36 shows the corrosion resistance of several nickel-based alloys exposed to stagnant seawater.

Acids/Alkalis

Nickel and nickel-based alloys are generally resistant to corrosion in nonaerated and nonoxidizing acids. Since sulfuric acid is not considered to be an oxidizing acid up to a concentration of about 50-60 wt.%, for example, most nickel alloys are generally resistant to corrosion in this environment. The corrosion rate of these alloys, though, typically increases with increasing sulfuric acid concentration. Nickel and its alloys are generally resistant to corrosion in acrylic acid and fatty acids.

The presence of oxidizing agents in acids or the aeration of acids can significantly increase the corrosion rate of the nickel alloy. For instance, nickel alloys are typically resistant to HCl in low concentrations, but the presence of Cu2+ or Fe3+, for example, may increase the corrosion rate of these alloys considerably. Some alloys, however, offer a better resistance in acids with oxidizing agents present, than others. Alloys containing chromium are more resistant to these types of acids, such as nitric and chromic acids, while molybdenum additions tend to degrade the resistance of nickel to these acid environments. The uniform corrosion rates of some nickel-based alloys in several acid environments are given in Table 37.

Nickel is strongly resistant to corrosion in alkalis, but environmental contaminants can cause an increase in the corrosion rate. Nickel is not, however, resistant to ammonium hydroxide solutions. The corrosion resistance of nickel alloys in alkalis tends to decrease with decreasing
nickel content.