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High temperature corrosion of iron occurs on passenger automobiles in areas such as exhaust headers, pipes and bolts that reach temperatures of about 500°C. The iron is in the form of steel or stainless steel and the corrosion is in the form of rust or scale.

What would occur if 99.9% pure metallic zinc (or aluminum) were brought into tight contact with iron in either its clean, rusted or scaled state and heated to normal exhaust temperatures in the presence of moisture and salts?

For example, exhaust flanges are clamped together using nuts and bolts and the bolts are long enough to allow a second nut to be installed. A zinc (or aluminum) washer is installed on the bolt and a second nut is used to crush it into the already-installed nut. The iron-oxide bolt / zinc or aluminum washer / iron bolt sandwich is heated in the presence of salts and moisture.

Would the zinc melt and drip away? Would such an assembly provide any cathodic protection against rust or scale? Would the preexisting flaky scale be converted into a blob of solid iron. Would a violent thermal reaction occur between the zinc and iron oxide and the exhaust burst into flames?

CLARIFICATION: the purpose of this question is to understand the likely interaction between Zn/Al and iron oxide/iron and uses exhaust pipes as a practical example. Specifically:

  • At the moment the Zn melts, would it be expected to wick into rusted areas and reduce the rust back into iron and produce a coating of zinc oxide?
  • Would an Al washer crushed between two rust-prone nuts provide any protection against rusting, especially at high temperatures, in the small area around the Al washer?
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Zinc and aluminum both melt. Aluminized steels area available. But the standard 13 % chrome SS does so well it is not used much any more. I have never seen corrosion problems on SS exhausts with well over 100,000 miles.

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