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 Uniform corrosion The uniform thinning of a metal without any localized attack Corrosion does not penetrate very deep inside. Familiar example is the rusting of steel in air Factors that can cause corrosion

Corrosion types Uniform corrosion Corrosion can proceed in a dry environment without any moisture if traces of sulfur compounds or H2S or other pollutants are present in the air.

Corrosion types Uniform corrosion

Corrosion types Uniform corrosion Effect of humidity Corrosion can be caused in the atmosphere when about 70% of the humidity is present. In the presence of such humidity, an invisible thin film of moisture is formed on the surface of a metal.

Corrosion types Uniform corrosion Water layer If visible water layers are formed on the metal surface, corrosion initiates. Splashing of seawater, rain and drops of dew provide the wet environment. Dew Formation If the dew becomes acidic, due to the presence of SO2, it increases the rate of corrosion.

Corrosion types Uniform corrosion Mechanism of uniform corrosion In atmospheric corrosion, a very thin layer of electrolyte is present. Corrosion products are formed close to the metal surface.

Corrosion types Uniform corrosion At the anodic area Mechanism of uniform corrosion At the anodic area At the cathodic area The OH ions react with the Fe++ ions produced at the anode

Corrosion types Uniform corrosion Mechanism of uniform corrosion With more access to oxygen in the air, Fe(OH)2 oxidizes to Fe(OH)3 and later it loses its water Ferrous hydroxide is converted to hydrated ferric oxide or rust by oxygen

Corrosion types Uniform corrosion Tarnishing of silver ware Example of uniform corrosion Tarnishing of silver ware Tarnishing of electrical contacts Rusting of steels in open air Corrosion of offshore drilling platforms Failure of distillation columns Corrosion of underground pipes Corrosion of heat exchanger tubes

Corrosion types Galvanic corrosion Definition Occurs when two metals with different electrochemical potentials or with different tendencies to corrode are in metal-to-metal contact in a corrosive electrolyte. When two metals with different potentials are joined, such as copper (+0.334 V) and iron (−0.440 V), a galvanic cell is formed. The driving force for corrosion is a potential difference between different materials

Corrosion types Galvanic corrosion Mechanism

Corrosion types Galvanic corrosion Mechanism The positive ions (Fe++) flow from the anode (iron) to cathode (copper) through the electrolyte, which is water. The hydrogen ions (H+) are discharged at the copper cathode, and ultimately hydrogen is released. The Fe++ ions travel towards the cathode and OH− towards the anode. They combine to form insoluble iron hydroxide, Fe(OH)2.

Corrosion types Galvanic corrosion Factors affecting galvanic corrosion Position of metals in the galvanic series. The magnitude of galvanic corrosion primarily depends on how much potential difference exists between two metals. the metals selected should be close to each other in the galvanic series to minimize galvanic corrosion. Example: aluminum should not be joined to steel, as aluminum being more active would tend to corrode

Corrosion types Galvanic corrosion The nature of environment. Factors affecting galvanic corrosion The nature of environment. Consideration must be given to the environment that surrounds the metal. Water containing copper ions, like seawater, is likely to form galvanic cells on a steel surface of the tank

Corrosion types Galvanic corrosion Factors affecting galvanic corrosion Area, Distance and Geometric Effects. The anode to cathode area ratio is extremely important as the magnitude of galvanic corrosion is seriously affected by it

Corrosion types Galvanic corrosion Factors affecting galvanic corrosion Area, Distance and Geometric Effects. The solution conductivity varies inversely with the length of the conduction path. If two different metals are far away from each other, there would be no risk of galvanic corrosion, because of very little current flow. As current does not flow around the corners, the geometry of the circuit affects the degree of galvanic corrosion

Corrosion types Galvanic corrosion Example of galvanic corrosion Galvanic corrosion of steel pipe with brass fittings. Galvanic corrosion of the body of the ship in contact with brass or bronze propellers. Galvanic corrosion between the tubes and the tube sheet in heat exchangers. Galvanic corrosion of steel coated with copper due to the defects in copper coating

Corrosion types Crevice corrosion Caused by the deposition of dirt, dust, mud and deposits on a metallic surface or by the existence of voids, gaps and cavities between adjoining surfaces. Presence of narrow spaces between metal-to-metal or non-metal to metal components Presence of cracks, cavities and other defects on metals Deposition of biofouling organisms and similar deposits Deposition of dirt, mud or other deposits on a metal surface

Corrosion types Crevice corrosion The conventional steels, like SS 304 and SS 316, can be subject to crevice corrosion in chloride containing environments Factors affecting crevice corrosion

Corrosion types Crevice corrosion Effect of bulk concentration

Corrosion types Crevice corrosion Mechanism Most of the mechanism is based on certain type of concentration cells. a difference in metal ions exists between the crevice and outside, hence, a corrosion cell is formed. A high concentration of oxygen on the surface outside the crevice and a low oxygen concentration inside a crevice creates a differential aeration cell, which initiates crevice corrosion

Corrosion types Crevice corrosion Mechanism

Corrosion types Crevice corrosion Mathematical model for crevice corrosion Developed by Oldfield and Sutton suggested four stages of crevice corrosion

Corrosion types Crevice corrosion Mathematical model for crevice corrosion Stage 1- the initial reaction when stainless steel is placed in a oxygenated neutral chloride solution The solution in the crevice becomes slowly depleted in oxygen

Corrosion types Crevice corrosion Mathematical model for crevice corrosion Stage 1 Oxygen is depleted and the solution inside becomes deoxygenated The metal ion concentration is increased

Corrosion types Crevice corrosion Mathematical model for crevice corrosion Stage 2- the cathodic reduction of oxygen proceeds outside the crevice and slow dissolution of the metal takes place inside the crevice

Corrosion types Crevice corrosion Mathematical model for crevice corrosion Stage 3- accelerated corrosion takes place due to the breakdown of the passive film

Corrosion types Crevice corrosion Mathematical model for crevice corrosion Stage 4- the crevice corrosion continues to propagate. Example of crevice corrosion - Development of leaks at the rolled joint of the pipe and pipe bottom in a heat exchanger. - Crevice corrosion of a tubing in a hydraulic oil cooler.

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