Research And Insight

Corrosion

Corrosion can occur on external and internal surfaces of pipes, tanks and radiators. In the design phase various issues have to be considered such as pipe material, insulation, medium, humidity and air purging etc.

Internal corrosion can occur if there is oxygen in the pipe or if there is very fast flow in the fittings. To prevent corrosion, system speeds should not be too high, and air must be prevented from getting into the system, e.g. do not use pipes into which air can diffuse. Air vents can be added if required as this will limit the amount of oxygen in the water.

External corrosion occurs when a steel pipe is fitted in a humid environment or when condensation occurs on the surface of a pipe used to transport a cold medium. Thus, the outer surfaces of cooling system pipes need to be protected from corrosion or to be made from non-corrosive materials such as plastic. Because air can diffuse through plastic, plastic pipes should not be used together with steel pipes.

Corrosion can occur in a tank or radiator when slurry precipitates in the bottom or when air accumulates in the top.

Crevice corrosion is a localised form of corrosion attack. Generally, metallic corrosion involves the degradation of an exposed metal surface. Corrosion occurs in various forms ranging from uniform attacks over the entire surface to severe local attacks.

Crevice corrosion starts more easily than for example pitting corrosion, another form of localised corrosion. Crevice corrosion occurs at narrow openings or spaces between two metal surfaces or between metals and non-metal surfaces. It is usually associated with stagnant conditions in the crevice. Crevices, such as those found at flange joints or at threaded connections, are thus often the most susceptible to corrosion attacks.

Corrosion fatigue describes a situation where a metal is exposed to both mechanical fatigue and corrosion simultaneously. Pure mechanical fatigue describes a condition where a material can fail as a result of a cyclical load far below the ultimate tensile strength.

If the metal is exposed to both a corrosive environment and mechanical fatigue simultaneously, the failure can take place at an even lower stress level and after a shorter time. Contrary to pure mechanical fatigue, there is no fatigue limit in corrosion-assisted fatigue.

Intergranular corrosion occurs at grain boundaries. Intergranular corrosion is also called inter-crystalline corrosion. As the name implies, intergranular corrosion occurs at grain boundaries. Intergranular corrosion is also called inter-crystalline corrosion.

Typically, this type of corrosion occurs when chromium carbide precipitates at the grain boundaries during the welding process or in connection with insufficient heat treatment. A narrow region around the grain boundary may therefore deplete in chromium and become less corrosion resistant than the rest of the material. This is unfortunate because chromium plays an important role in corrosion resistance.

Examples of metals that are subject to intergranular corrosion:

  • Stainless steel, which is insufficiently welded or heat-treated
  • Stainless steel EN 1.4401 (AISI 316) in concentrated nitric acid

Grundfos supplies corrosion-resistant pumps, for example built with Titanium, for extreme environments.