WHEN carriers are considering purchasing a tank trailer, they should take into consideration materials with increased resistance to corrosion — and specify linings and coatings. Fiberglass reinforced plastic cargo tanks also can be specified for hauling high-corrosive products.
Those were some of the suggestions given by Peter Weis of Polar Corp, who talked about three types of corrosion to a cargo tank — general, pitting, and intergranular — and ways to prevent them.
General corrosion attacks the entire metal surface, causing thinning or rusting, and can be aggravated by products such as sulfuric, phosphoric and nitric acids and sodium hydroxide. High temperatures, concentration of the acid, moisture, and fumes are also contributors.
General corrosion can be combated by specifying adequate corrosion allowance. He noted that yearly corrosion rates can be predicted by using available sources.
Another problem comes with pitting that can be caused from products containing chlorides, iodides, or bromides. Pitting occurs with the breakdown of the passive chromium oxide layer and can be aggravated by product acidity, imbedded iron in the presence of chlorine, surface defects in the material, elevated temperatures, and delayed tank cleaning.
Passivation can help prevent pitting because it smoothes the surface and removes contaminants and iron. The surface should be finished smooth and tank cleaning should be prompt.
Intergranular corrosion occurs from an attack of material adjacent to welds in the heat-affected zone, not in the weld itself. Offending products include oxidizing acidic solutions.
The corrosion is caused by the weld heat causing chromium to react with carbon in the stainless steel. That creates chromium carbides at the grain boundaries, which lowers the chromium content in the areas. The result is a poor oxide layer.
How to prevent intergranular corrosion? Use “L” grade stainless steel and welding wire, and titanium stabilized material.
“Note that increased density pitting on the weld versus the base metal is mainly related to surface finish and how the finish affects the passive layer,” he said. “This is sometimes mistaken for intergranular corrosion.”