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Corrosion and Erosion
Corrosion and erosion are unremitting enemies; often they are encountered simultaneously and the problems they pose demand consideration when deciding the choice of material. Accelerated corrosion or corrosion-erosion may arise from any of the following causes.
- Galvanic action when dissimilar metals are associated.
- Local deposits or transfer of small metals fragments e.g. weld spatter in steel pipes, by sulphur or sulphides in weld spatter in steel pipes, by sulphur or sulphides in polluted waters, gassing in hot tubes of coolers, fouling and broken cathodic films in pipe bores.
- High water speeds, entrained air, cavitation and turbulence caused by e.g. protuberences, tight bends, abrupt change of direction or pipe sectional areas. (This form of erosion is usually encountered as pitting)
- Dezincification or corrosion-stress cracking of brass and some so called 'bronzes'. Corrosion stress cracking is a phenomenon associated chiefly with brass tube stressed by expanding or by working in the unsoftened condition; in corrosive fluids, e.g. sea water, splitting occurs suddenly, even violently.
- Inadequate protection of steel, pin holes or discontinuities in protective linings. These should always be carried over the flange faces.
Engineers should be familiar with the galvanic series. In this, the more noble metals are placed after the less noble thus: zinc, aluminium, carbon steels, cast iron, lead-tin alloys, lead, brass, copper, bronze, gun metal, copper-nickel iron, monel. A metal in contact with one occurring latter in the series, e.g. steel in contact with copper, may corrode rapidly in sea water or even in condensate.
The action is galvanic and sacrificial anodes give protection when attached in a manner giving good electrical bonding. Examples are soft iron plates in condenser water boxes and zinc plates around the propeller aperture, adjacent to high tensile brass propellers. Basically, brass is an alloy of copper and zinc and bronze an alloy of copper and tin. In both cases there may be additions of other metals and there is some confusion of nomenclature. Some high tensile brasses are called 'bronze' and the practice has prevailed for so long as to be accepted.