Nonelectrochemical Principle of Corrosion Protection

Pitting corrosion, on the other hand, occurs without gross delamination of the coating during the normal duration of salt spray tests (e.g., 1-3 months), and only removal of the coating after the salt spray tests can reveal the presence of pits. Pitting 

In order to compare the relative extent of the damaged surface corrosion and of the pitting corrosion, the percentage of the corroded area in the tested surface is used, and summarized in Table 28.2. It is important to note that pitting corrosion was found only on chromate conversion-coated controls (one with chromated spray primer and another with E-coat). It is also important to note that E-coat directly 

Substrates Coating systems Pitting corrosion (%) Along the scribe (%)  

Substrate Pretreatment Tape test rating El E2 NMP time (min)  

A nonchromated, water-borne primer applied to [2B] alloy samples, with the appropriate surface preparation and plasma deposition of an ultrathin plasma polymer, was also compared to controls prepared by depositing a chromated primer on chromate conversion-coated A1 substrate. The same comparison was also performed for IVD Al-coated 2024-T6 substrates (pure aluminum is deposited by ion vapor deposition process on aluminum alloy 2024-T6). In the latter case, the primer could not be removed from the IVD Al-coated panels that were treated with the plasma polymer prior to spray primer application. It is interpreted that the water-borne spray paint penetrates into the column structure of the top surface of the IVD Al-coated substrates when the surface energy was modified by the application of a plasma polymer. This effect could be viewed as interactive coating with a porous surface. 

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