Effect of pH and Halides on Metal Corrosion

Effect of pH on metal corrosion

The effect of pH on metal corrosion is complicate. It is generally believed that extreme pH range, i.e. strong acidic and alkaline, is favourable for corrosion. In moderation pH range, i.e. weak acidic and weak alkaline, the effect is more metal specific

Steel

Changing of pH into the moderate high range, i.e., 8<pH<10.5, corrosion is usually less aggressive. The iron oxides are less soluble in such alkaline solution. Steel corrosion is aggressive in the acidic pH range. This explains the water condensation, such as steam condensation or condensed water in the top of pipe, is very aggressive. There is minimum mineral in such condensation, therefore, pH of the water is not buffered and easily changed by absorbing acidic corrodants (CO₂, H₂S, etc)

Tungsten

Tungsten oxide is active in alkaline solution and stable in weak acid solution. Therefore, Tungsten  is more likely to be corroded aggressively in alkaline environment.

Molybdenum

Molybdenum oxide is also active in alkaline solution and stable in weak acid solution. Therefore, Molybdenum is more likely to be corroded aggressively in alkaline environment as well.

Aluminum, Tin and Zinc

The oxides of aluminum, tin and zinc are active in wide range of pH, from acidic to alkaline. Therefore, Al, Sn and Zn are more likely to be corroded in wide pH range.

Effect of water on metal corrosion

It is normally believed that electrochemical corrosion process would not occur if there is no water. However, there are some exemptions.

Titanium

Oxygenated water reacts with titanium to form a protective titanium oxide layer. The dense film is chemically inert, which is protective against otherwise aggressive chlorine.

However, hot halides (NaCl, NaBr, NaI) will cause Titanium alloy stress corrosion cracking in the presence of water at elevated temperature (343 ~ 400^oC). This was found in aircraft gas turbine over oceans: the salt  and moisture in atmosphere were picked up by jet engine in normal operation.Excluding of water will prevent such corrosion damage.

Effect of Halides on metal corrosion

Halides usually have detrimental effect on the passivation film on the metal. As a result of breach passivation film, pitting corrosion is initialised and may further develop into stress corrosion cracking under certain conditions. However, there are some interest exemptions.

Corrosion of Steel in Sulphuric acid

Halides of fluorine, bromine, chlorine and iodine help to inhibit the corrosion of steel in sulphuric acid.