Corrosion process and control (TKK-2289) 15/16 Semester genap Corrosion process and control (TKK-2289) Instructor: Rama Oktavian; Vivi Nurhadianty. Email: rama.oktavian86@gmail.com Office Hr.: T. 11-12, Th. 08-10; 13-15, F. 08-10; 13-15

Corrosion types Pitting corrosion Localized corrosion of a metal surface where small areas corrode preferentially leading to the formation of cavities or pits. It causes failure by penetration with only a small percent weight-loss of the entire structure. the most conducive environment for pitting is the marine environment

Corrosion types Pitting corrosion Conditions for pitting to occur: Breaks in the films or other defects The presence of halogen ions Stagnant conditions in service

Corrosion types Mechanism of Pitting corrosion the formation of an anode is a prerequisite  a local corrosion cell is developed. lack of homogeneity at the metal corrosive interface destruction of a passive film deposit of debris or solids on the metal surface formation of an active–passive cell with a large potential difference

Corrosion types Pitting process The formation of anodic sites by disruption of the protective passive film. The metal dissolution takes place at the anode. Pitting is, therefore, initiated. Due to the continuing metal dissolution, an excess of positive ions M+ is accumulated in the anodic area The presence of H+ ions and chloride content, prevents repassivation The increase in the rate of dissolution at the anode increases the rate of migration of the chloride ions The process continues until the metal is perforated.

Corrosion types Pitting process

Corrosion types Main step of Pitting process

Corrosion types Factors affecting pitting corrosion Chloride ions concentration Alloying elements Effect of electrolyte composition Effect of pH Effect of temperature

Corrosion types Influence of alloying elements in pitting corrosion of aluminum Chromium. It is usually added to Al–Mg alloy and Al–Mg–Zn alloys in amounts of 0.1–0.3%. Copper. It reduces the corrosion resistance of aluminum alloys and increases the susceptibility to pitting Iron. It exists in the form of FeAl3 and promotes pitting of aluminum alloy Magnesium. It provides substantial strengthening with good ductility in addition to excellent corrosion resistance

Corrosion types Evaluation of pitting damage Conventional weight-loss techniques cannot be used to evaluate pitting. What matters in pitting is the depth of the deepest pit The development of pit depths follows a time function:

Corrosion types Pitting corrosion

Corrosion types Pitting corrosion An example of corrosion damages with shared responsibilities was the sewer explosion that killed 215 people in Guadalajara, Mexico, in April 1992. Besides the fatalities, the series of blasts damaged 1,600 buildings and injured 1,500 people. *Sewer: an artificial conduit, usually underground, for carrying off waste water and refuse, as in a town or city.

Corrosion types Pitting corrosion

Corrosion types Pitting corrosion THROUGH PITS SIDEWAY PITS Subsurface Narrow, deep Shallow, wide Undercutting Elliptical Horizontal grain attack Vertical Grain Attack

Corrosion types Intergranular corrosion a form of localized attack on the grain boundaries of a metal or alloy in corrosive media, which results in the loss of strength and ductility. Intergranular corrosion is less dangerous than stress corrosion. As corrosion proceeds along the grain boundaries, the grains become weaker particularly at the grain boundaries and they eventually disintegrate

Corrosion types Intergranular corrosion Metals are crystalline materials. They consist of grains All metals and alloys are joined together by grain boundaries

Corrosion types Technical term of steel Sensitization When steels are slowly cooled through the range 550–850◦C Stabilized steel Steels containing stabilizers, such as titanium and niobium, which form carbides Chromium depletion When the concentration of chromium adjacent to the grain boundaries becomes less compared to its concentration away from the grain boundaries Quenching cooling rapidly by quenching in water

Corrosion types Mechanism of intergranular corrosion Sensitization in the temperature range 550–900◦C Diffusion of carbon to a grain boundary and formation of carbide Weakening of the grain boundary and disintegration of the grains

Corrosion types Mechanism of intergranular corrosion

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