CORROSION FORMS

                               FORMS OF CORROSION

The following is a brief review of the major types of corrosion;

UNIFORM CORROSION:

This common form of corrosion is characterized by uniform attack over the entire exposed surface corrosion product may form a protective layer on the metal that decreases the rate of corrosion , for example , magnetite on the carbon steel in the H T S. The original criteria normally account for the extent of material wastage.

Chemical control and the use of protective coatings are most commonly used protection against uniform corrosion 

GALVANIC CORROSION:

The galvanic consists of two dissimilar metals in an electrolyte and also having external electrical contact with each other. At least one of the metals must have a positure oxidation potential. A potential difference due to the different oxidation potential of the metals causes electrons to flow in the circuit. Oxidation (corrosion) occurs at the anode where metals are removed. Deductions (of electrolyte) occur at the cathode. Galvanic corrosion occurs when two dissimilar metals are in electrical contact in an electrolyte reaction. 

PITTING END CREVIC CORROSION 

Pitting and crevice corrosion are similar electro-chemical corrosive actions with metal loss or destruction occurring at localized anodic areas. The electrochemical action does not depend upon contact between dissimilar metals, but on localized differences in electrolyte concentration, i.e. the electrolyte contains a “hostile” ion/element at two distant concentrations. This situation maybe prompted by low-velocity, stagnant type conditions. 

Pitting corrosion is commonly associated with deposit or scale creating between different concentrations of the hostile substances in the electrolyte.

Crevice corrosion describes the location e.g. bolts, gasket, metal contact point where this type of metal attack occurs. 

STRESS CORROSION CRACKING (CSS)

 Stress corrosion cracking is the formation of cracks when localized corrosion has combined with steady tensile strength in the metal to cause the damage. The effect has been seen in low pressured turbine discs and blade rots and also in boiler tubes. The hostile electrolytic environment can attack particular metals or alloys. For example, chloride and stainless steel. Excessive SCC can cause failure, typically sudden without warning. 

 

EROSION CORROSION:

Erosion corrosion is the acceleration of corrosion owing to relative movement of the corrosive fluid and metal surface.

            It is characterized by grooves, waves and valleys in the metal surface, and short time periods to unexpected failures. Erosion corrosion is promoted by high fluid velocity, turbulent low and the impingement of those high velocity fluids on metal surface for example, at elbows in pipelines. 

MICROBIOLOGICALLY INDUSED CORROSION:

MIC is corrosion involving the action of bacteria on metal surfaces, most commonly in stagnant water. Slime-formation bacteria are aerobic and thrive in most cooling water systems. As they metabolize dissolve oxygen from water, they create an anaerobic bacteria can attack metal surfaces.