Question

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? 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?

Answer 1

When zinc or aluminum are in tight contact with iron and heated, they can provide cathodic protection against rust or scale. Zinc or aluminum can melt and potentially drip away, and a violent thermal reaction could occur between the metals and iron oxide, possibly causing flames. However, the protective effect may be limited to the small area around the metals.

Step-by-step explanation:

When 99.9% pure metallic zinc or aluminum is brought into tight contact with iron and heated to normal exhaust temperatures in the presence of moisture and salts, several reactions can occur. First, the high temperature could cause the zinc or aluminum to melt and potentially drip away, depending on the specific temperatures involved. Second, the zinc or aluminum could provide some cathodic protection against rust or scale by acting as a sacrificial anode and being oxidized instead of the iron. Third, if the iron is rusted or scaled, a violent thermal reaction could occur between the zinc or aluminum and the iron oxide, possibly causing the exhaust to burst into flames.

At the moment the zinc melts, it could be expected to wick into rusted areas and reduce the rust back into iron while producing a coating of zinc oxide. However, it is important to note that the protection provided by the zinc or aluminum washer in this setup may be limited only to the small area around the washer and may not fully prevent rusting, especially at high temperatures.