Which of the following is not considered a significant factor in atmospheric corrosion?

Atmospheric corrosion primarily occurs on materials exposed to environmental conditions outside of a controlled space. In this context, humidity levels, temperature fluctuations, and the presence of contaminants are all significant factors influencing the rate and extent of corrosion.

Humidity levels play a crucial role because moisture in the air can facilitate the electrochemical reactions that lead to corrosion. Higher humidity generally accelerates the process. Temperature fluctuations can also contribute by affecting the moisture levels and conditions under which corrosion occurs. For instance, rising temperatures may increase evaporation and condensation cycles, which could enhance the corrosion process.

The presence of contaminants, such as salts, pollutants, or other chemicals, can intensify corrosion by providing additional pathways or enhancing the corrosive environment. These factors all contribute significantly to the mechanisms and rate of atmospheric corrosion.

Vessel operating pressure, however, is more relevant to the structural integrity and stress factors that affect pressure vessels under their operational conditions. While atmospheric pressure can have an indirect relationship with corrosion in certain contexts, the specific operating pressure within the vessel does not significantly impact corrosion from atmospheric exposure. Thus, it is the least relevant factor compared to the other options, which directly relate to atmospheric conditions.