Effect of Velocity on Corrosion in Natural Waters

7.2.5 Effect of Velocity on Corrosion in Natural Waters

In natural fresh water, the pH is usually too high for hydrogen evolution to play an important role, and relative motion of the water at fi rst increases the corrosion rate by bringing more oxygen to the surface. At suffi ciently high velocities, enough oxygen may reach the surface to cause partial passivity. If this happens, the rate decreases again after the initial increase (Fig. 7.8 ). Should the velocity increase still further, the mechanical erosion of passive or corrosion - product fi lms again increases the rate. The maximum corrosion rate preceding passivity occurs at a velocity that varies with smoothness of the metal surface and with impurities

in the water. In the presence of high concentration of Cl − , as in seawater, passivity is not established at any velocity, and the corrosion rate increases without a

Figure 7.8. Effect of velocity on corrosion of mild steel tubes containing Cambridge water, 21 ° C, 48 - h tests [21] . [ Reprinted with permission from R. Russell et al., Ind. Eng. Chem. 19 , 65 (1927). Copyright 1927, American Chemical Society .]

130 IRON AND STEEL

Figure 7.9. Effect of velocity on corrosion of steel in seawater [22] .

decrease at any intermediate velocity (Fig. 7.9 ). The same behavior is expected at elevated temperatures that preclude the possibility of passivity by dissolved oxygen.

7.2.5.1 Cavitation – Erosion. If conditions of velocity are such that repeti- tive low - pressure (below atmospheric) and high - pressure areas are developed, bubbles form and collapse at the metal – liquid interface. This phenomenon is called cavitation. The damage to a metal caused by cavitation is called cavita- tion – erosion, or cavitation damage. The damage can be reproduced in the labora- tory using metal probes undergoing forced high - frequency mechanical oscillations normal to the metal surface. The surface becomes deeply pitted and appears to

be spongy (Fig. 7.10 ), or like a honeycomb - like structure [24] . Damage from this cause may be purely mechanical, as is experienced with glass or plastics or when damage to a metal occurs in organic liquids. The damage, however, may involve both chemical and mechanical factors, particularly if protective fi lms are destroyed, allowing corrosion to proceed at a higher rate. The part played by chemical factors is apparent, for example, from increased metal loss in laboratory tests using seawater compared to fresh water.

Cavitation – erosion occurs typically on rotors of pumps, on the trailing faces of propellers and of water - turbine blades, and on the water - cooled side of diesel - engine cylinders [25] . Cavitation attack is usually controlled by design and mate- rials selection [26] . Damage can be reduced by operating rotary pumps at the highest possible head of pressure in order to avoid bubble formation. For turbine blades, aeration of water cushions the damage caused by collapse of bubbles. There is a wide range of polymers with good resistance to cavitation – erosion and excellent resistance to corrosion — for example, high - density polyethylene has a cavitation – erosion resistance similar to that of nickel - based and titanium alloys

[26, 27] . Since 18 – 8 stainless steel is one of the relatively resistant alloys, it is used as a facing for water - turbine blades. To reduce damage of diesel - engine cylinder liners, addition of inhibitor to the cooling water has proved effective [28] .

AQUEOUS ENVIRONMENTS

Figure 7.10. Cavitation–erosion damage to cylinder liner of a diesel engine [23]. ( Copyright NACE International, 1950 .)