Graduate Thesis Or Dissertation
 

A mechanism for filiform corrosion of aluminum

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  • A mechanism for filiform corrosion of aluminum is proposed based on experimental evidence and the known aqueous chemistry of the trivalent aluminum ion. Experimental methods utilized include autoradiographs with Cl³⁶; pH paper determination of the filiform head pH; activator concentration and activation period as they affect filiform growth. The pH of the active filiform head was determined to be 1.5 - 2.0 pH units. An activator concentration of 0.1 N HC1 was the minimum amount necessary to initiate good filiform growth and an activation period of at least 180 seconds was also necessary to initiate good filiform growth. Autoradiographs showed a small amount of Cl³⁶ remaining in the corrosion product with the heaviest concentration present in the filiform head. Activation occurs at a break or weak point in the film, Due to osmotic pressure, water enters this area causing film separation and an electro-chemical corrosion cell is formed. Aluminum is dissolved at the anode, going into solution as the trivalent aluminum ion. This ion hexa-hydrates, accounting for the low pH value in the head due to its acidic nature. Simultaneous products of hydrogen gas and hydroxide ion are formed at the cathode utilizing water and oxygen in the process. Due to mixing and convection within the cell, the Al⁺⁺⁺ precipitates as Al(OH)₃ at the transition boundary when the solubility product is exceeded and the corrosion product is deposited with a subsequent loss of water to the atmosphere. Insufficient relative humidity causes the boundary to catch the head and the cell dies while an excess of relative humidity will result in blisters and general corrosion. Excess oxygen will also increase the rate and extent of filiform growth.
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