Which combination will result in more than one form of corrosion within a system?

• A. Metals used in structures/equipment and a wide range of environments
• B. A single metal in a single, uniform environment
• C. Two identical alloys in a single environment
• D. A nonmetallic component in air

Answer: (A)

The main idea here is that multiple corrosion forms show up when different metals are used and the environment varies across the system. Dissimilar metals in contact in the presence of an electrolyte create galvanic cells, where the more anodic metal corrodes faster. At the same time, parts of the system exposed to different oxygen levels, moisture, or crevices can develop differential aeration or crevice corrosion. Put together, metals in a range of environments set up several distinct electrochemical conditions, so more than one corrosion mechanism can operate simultaneously in the same system.

If you have a single metal in a uniform environment, you’ll typically see one dominant corrosion form. Two identical alloys in a single environment behave similarly and don’t create galvanic couples, so you’re unlikely to get multiple forms. A nonmetallic component in air isn’t undergoing metal corrosion, so it won’t produce the variety of metal corrosion mechanisms discussed here.