Metal Samples MS2500L, Руководство оператора

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Appendix C: Effects of Process Variables
1. Temperature
As a general rule, increasing temperature increases corrosion rates. This is due to a combination of
factors: first, the common effect of temperature on the reaction kinetics themselves and second, the
higher diffusion rate of many corrosive by-products at increased temperatures. This latter action delivers
these by-products to the surface more efficiently.
Occasionally, the corrosion rates in a system will decrease with increasing temperatures. This can occur
because of certain solubility considerations. Many gases have lower solubility in open systems at higher
temperatures. As temperatures increase, the resulting decrease in solubility of the gas causes corrosion
rates to go down.
2. pH
Corrosion rates almost always increase with decreasing pH (increasing acidity).
This is a direct result of increasing the concentration of an aggressive ion (H-) and
increasing the solubility of most potentially corrosive products.
3. Oxygen Concentration
Oxygen’s role in corrosion is as an aggressive gas or oxidizing agent. As its con-
centration increases, corrosion rates increase until the rates of diffusion to the sur-
faces reach a maximum. The same principles apply to most other oxidizing agents,
such as Cl2, H. Br2.
4. Fluid Velocity
The dependence of corrosion rate on fluid velocity is complex. In general, the
higher the velocity, the higher the corrosion rate. At very low velocities, even zero,
there are diffusion effects that can cause corrosion. As fluid velocities increase
from stagnant to moderate values, the corrosion rates increase. Then, as the limit of
diffusion at a particular temperature is reached, further increases in velocity have
little effect on the corrosion rate. At some point, however, the velocity reaches such
high values that the surface film of the metal begins to be damaged. At these veloci-
ties, the corrosion rates resume increasing with the higher velocities.