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#064 Water Corrosion

Category : Corrosion - Corrosion Protection
December 3, 2010

Steel corrosion progress by water depends on the amount of oxygen dissolved in the water. The dissolved oxygen concentration supplied by atmosphere at normal temperatures is very low of 8~10ppm, so the speed of corrosion progress will depend on dissolved oxygen diffused from water.

If no foreign objects are adhering on the steel surface, the speed of steel corrosion in static fresh water is 0.4mm/year based on dissolved oxygen concentration and diffusion rate. In actuality, it is said that formation of rust would slow down the rate to 0.1mm/year.

Not as common in Japan where surface water flow are rapid due to the country's mountainous terrain, but in Europe and the Americas where the lands are flatter and the surface water flows slowly the water hardness can be high containing much calcium from the ground and rocks. When steel comes in contact with hard water, a surface layer of calcium carbonate is formed inhibiting the dissolved oxygen, and significantly delaying the corrosion process.

It is said that water influences on corrosion in Japan is relatively insignificant within pH of 5~9, and concentrations of chloride and sulfate ions are also insignificant in static water condition. This is indicative of the fact that dissolved oxygen concentration level determines the rate of corrosions.

When there is a significant fluid flow, the amount of dissolved oxygen supply would increase and the corrosion rate would also increase as the flow rate increases. If the flow increases beyond certain rates, dissolved oxygen supply becomes excessive and cannot all be consumed for steel dissolution, and passive layer formation would begin, then the corrosion rate would decrease. This also occurs when the water temperature decreases and dissolved oxygen concentration increases.

The effects of chloride and sulfate ions in fresh water on steel corrosions, regarding the fluid flow conditions for passive surface layer formation and post passive layer formation corrosion rates, become larger as ion concentrations increase. In water high in chloride ion concentration such as sea water, passive layer formation does not occur even if the flow rate is increased. Generally, chloride ions are known to be a corrosion promoting agent, but in the case of water immersed carbon steels the negative effects apply only when the flow rate is at over Critical Flow.

Steel corrosion rate in water increases up to 80°C, but begins to decrease when that is exceeded. This is due to a conflicting effect on the corrosion rate of dissolved oxygen diffusion rate increase by the increased temperature, while oxygen solubility decreases.

Stainless steels and aluminum do not corrode in fresh water, in principle, but Pitting Corrosions and Crevice Corrosions will become of issues when chloride ions exist in the water.