Introduction to Corrosion

Corrosion is the destructive attack of a material by reaction with its environment. Almost all material is expected to deteriorate with time when exposed to a particular environment. Corrosion is a perfectly natural process – as natural as water flowing downhill. The driving force that causes metals to corrode is a natural consequence of their temporary existence in metallic form. To reach this metallic state from their natural state in the form of various chemical compounds (ores), it is necessary for them to absorb and store up the energy required to release the metals from their original compounds. The amount of energy required and stored varies from metal to metal.

The energy required is relatively high for metals such as magnesium, aluminum, and iron, and relatively low for metals such as copper and silver. The most common iron ore, hematite, is an oxide of iron (Fe2O3). The most common product of the corrosion of iron – rust, has the same chemical composition. The energy required to convert iron ore to metallic iron is returned when the iron corrodes to form the same compound – only the rate of energy change is different.

Destruction by corrosion takes many forms, depending on the nature of the metal or alloy; the presence of inclusions or other foreign matter at the surface; the homogeneity of its structure; the nature of the corrosive medium; the incidental environmental factors such as the presence of oxygen and its uniformity, temperature, and velocity of movement; and other factors such as stress (residual or applied, steady or cyclic); oxide scales (continuous or broken); porous or semiporous deposits on surfaces, built-in crevices; galvanic effects between dissimilar metals; and the occasional presence of stray electrical currents from external sources. Except in rare cases of a grossly improper choice of material for a particular service, or an unanticipated drastic change in the corrosive nature of the environment or complete misunderstanding of its nature, failures of metals by rapid general attack are not often encountered. Corrosion failures are more often localized in the form of pits, intergranular corrosion, attack within crevices, etc.

Costs of Corrosion
Although the costs attributed to corrosion damages of all kinds have been estimated to be of the order of 3 to 5 percent of industrialized countries’ gross national product (GNP), the responsibilities associated with these problems are spread to industry, government, producers and manufacturers.

The elements of its costs include:
Capital Costs
– Replacement of equipment and structures
– Excess capacity
– Redundant equipment

Control Costs
– Maintenance and repair
– Corrosion control

Design Costs
– Materials of construction
– Corrosion allowance
– Special processing

Corrosion Control
Corrosion control is achieved by using corrosion-resistant materials and designs, and by using protective systems, devices, and treatments. Ordinary iron does not naturally form an effective barrier; its rust permits oxygen and moisture to penetrate and continue rusting. Thus, unless precautions are taken, failure will eventually occur. Precautions to prevent iron and its alloys from corroding constitute a major effort in corrosion control.

Often painting, electroplating, and other measures are used to form artificial protective layers over the iron surface and thus prolong its useful life. Some metals, like stainless steel, titanium, or aluminum, are frequently left unpainted. This is not because these metals are inert, but because the oxygen of the air helps develop a protective surface layer of chromium oxide, titanium oxide, and aluminum oxide. If such coatings did not form naturally over the entire surface of these normally corrosion-resistant metals, they would be susceptible to corrosive attack. Some environments are more corrosive than others.

Factors include:

  • Temperature Effects – increasing temperature most often increases corrosion rates
  • Potential (Emf) Difference – when there is a difference in potential metals exposed to the same environment (such as zinc and steel in sea water) the metal higher in the series (zinc) will corrode and protect the lower one (steel)
  • Heat Treatment
  • Surface Condition – cleanliness of a surface and presence of foreign matter influences the initiation and rate of corrosion
  • Radiation
  • Environmental Impurities
  • Time
  • Stress
  • Under proper conditions, a higher rate of corrosion occurs when a material is subjected to a tensile stress

Handbook of Corrosion Engineering, Pierre Roberge, McGraw-Hill, 2000
Corrosion Basics – An Introduction, National Association of Corrosion Engineers, 1984
Uhlig’s Corrosion Handbook, Third Edition, R. Winston Revie, Ed. John Wiley & Sons, 2011

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