Page 74 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

52 INTRODUCTION AND FORMS OF CORROSION

Agitation or circulation of the electrolyte results in the increase in corrosion
because of: (i) destruction or prevention of the formation of a protective film (50);
(ii) increase in the rate of diffusion of aggressive ions because of the speed of the
fluid thereby decreasing the cathodic polarization and increasing the current density
as in the case of steels in the presence of oxygen, carbon dioxide, or bisulfate.
On the contrary, the speed of the fluid can decrease the corrosive attack by: (i)
improving the inhibition efficiency through the quick transport of inhibitor to the
metal–solution interface; (ii) by forming a passive layer for an active metal because
of greater supply of oxygen to the surface; (iii) by sweeping away the corrosive agent
(ionic species); (iv) by agitating and circulating at certain speeds of the medium and
avoid pitting and crevice corrosion (17).

1.7.15 Effect of Turbulence
At very high flow rates of fluids, corrosive attack occurs by the combined action of
erosion and corrosion. At high speeds, the abrasive action of the flow becomes severe.
Thus the speed of flow strongly influences the mechanisms intervening in corrosion.
The type of flow such as laminar or turbulent depends on the rate and quantity of
fluid transported and on the geometry and design of equipment. Straight line flow is
less damaging. Turbulent flow involves more intimate contact between the metal and
the fluid. Other factors such as edges, cracks, deposits, abrupt changes of section,
and other obstacles that disrupt laminar flow contribute to the turbulence and hence
impingement attack (17).

1.7.16 Galvanic Effect
The galvanic cell between two different metals can have serious effects in a flowing
system. In the case of 316 stainless steel and lead in 10% sulfuric acid under static
conditions there was no galvanic cell. On increasing the flow rate to 11.89 m/s the rate
of erosion–corrosion increased enormously because of the destruction of the passive
film by the combined effect of galvanic corrosion and erosion–corrosion (17).

1.7.17 Water Droplet Impingement Erosion
In liquid impingement erosion, with small liquid drops stroking the surface of the
solid at high speeds (as low as 300 m/s), very high pressures are experienced, exceed-
ing the yield strength of most materials. Thus, plastic deformation or fracture can
occur from a single impact, and repeated impacts lead to pitting and erosive wear
(60). Water droplet impingement causes pitting and may cause cavitation damage.
The damage may appear to be somewhat different from cavitation damage in duc-
tile materials. The cavities in the surface show a directionality that is related to the
angle of attack of the drop, as in erosion–corrosion. Steam turbines and water rotor
blades are most susceptible to this form of attack. In turbines, condensation of steam
produces droplets that are carried into rotor blades and causes surface damage. Rain