by Anita Pravitasari

Many factors have to be considered when selecting engineering materials, but for chemical process plant the overriding consideration is usually the ability to resist corrosion. The condition that cause corrosion can arise in a variety of ways. It is convenient to classify corrosion according to the manner in which it is manifest. Metallic corrosion is sometimes classified into six forms: uniform, galvanic, pitting, intergranular, erosion–corrosion, and stress corrosion. Hydrogen embrittlement is, in a strict sense, a type of failure rather than a form of corrosion; however, it is often produced by hydrogen that is generated from corrosion reactions.

Uniform Corrosion

General or uniform type of corrosion This term describes the more or less uniform wastage of material by corrosion, with no pitting or other forms of local attack. If the corrosion of a material can be considered to be uniform the life of the material in service can be predicted from experimentally determined corrosion rates. The corrosion rates will be dependent on the temperature and concentration of the corrosive fluid. An increase in temperature usually results in an increased rate of corrosion, though not always. The effect of concentration can also be complex. For example, the corrosion of mild steel in sulphuric acid, where the rate is unacceptably high in dilute acid and at concentrations above 70 per cent, but is acceptable at intermediate concentration.

Galvanic Corrosion

If dissimilar metals are placed in contact, in an electrolyte, the corrosion rate of the anodic metal will be increased, as the metal lower in the electrochemical series will readily act as a cathode. If metals which are widely separated in the galvanic series have to be used together, they should be insulated from each other, breaking the conducting circuit. Alternatively, if sacrificial loss of the anodic material can be accepted, the thickness of this material can be increased to allow for the increased rate of corrosion. The corrosion rate will depend on the relative areas of the anodic and cathodic metals. A high cathode to anode area should be avoided.

Pitting

Pitting corrosion on external pipe surface Pitting is the term given to very localized corrosion that forms pits in the metal surface. If a material is liable to pitting, penetration can occur in advance and corrosion rate data are not a reliable guide to the equipment life. Pitting can be caused by a variety of circumstances. Any situation that causes a localized increase in corrosion rate may result in the formation of a pit. A good surface finish will reduce this type of attack. Pitting can also occur if the composition of the metal is not uniform.

Intergranular Corrosion

Intergranular corrosion in sensitized stainless steels Intergranular corrosion is the preferential corrosion of material at the grain (crystal) boundaries. Though the loss material will be small, intergranular corrosion can cause the catastrophic failure of equipment. Intergranular corrosion is a common form of attack on alloys but occurs rarely with pure metals. The attack is usually caused by a differential couple being set up between impurities existing at the grain boundary. The classic example of intergranular corrosion in chemical plant is the weld decay of unstabilised stainless steel.

Erosion-corrosion

Errosion corrosion at a 90º change in direction The term erosion-corrosion is used to describe the increased rate of attack caused by a combination of erosion and corrosion. If a fluid stream contains suspended particles, or where there is high velocity or turbulence, erosion will tend to remove the products of corrosion and any protective film, and the rate of the attack will be markedly increased. If erosion is likely to occur, more resistant materials must be specified, or the material surface protected in some way. For example, plastics inserts are used to prevent erosion-corrosion at the inlet to heat-exchanger tubes.

Stress Corrosion

Corrosion rate and the form of attack can be changed if the material is under stress. Generally, the rate attack will not change significantly within normal design stress values. However, for some combinations of metal, corrosive media and temperature, the phenomenon called stress cracking can occur. This is the general name given to a form of attack in which cracks are produced that grow rapidly, and can cause early, brittle failure, of the metal. Stress corrosion can be avoided by selecting materials that are not susceptible in the specific corrosion environment; or, less certainly, by stress relieving by annealing after fabrication and welding.

Hydrogen Embrittlement

Hydrogen embrittlement is the name given to the loss of ductility caused by the absorption (and reaction) of hydrogen in a metal. It is of particular importance when specifying steels for use in hydrogen reforming plant. Alloy steels have a greater resistance to hydrogen embrittlement than the plain carbon steels.