What Does Poultice Corrosion Mean?
This is corrosion that results from the accumulation of dirt, among other forms of debris. When this happens, a highly corrosive condition is created.
In automobiles the most susceptible areas include inside frames, wheel wells and hem flanges within doors. These areas tend to remain wet, which results in a highly corrosive liquid from the moisture trapped in the poultice. Salts cause serious aggravation due to wet-dry cycling as well as accumulation. The areas most affected are regions where there are high levels of sulfates, sodium, chloride and calcium ions.
Corrosionpedia Explains Poultice Corrosion
In an aluminum window frame, water is trapped by debris and dust, thus becoming acidic. The acidic solution is called aluminum hydroxide, which perforates the window frame. This leaves white deposits on the bare and unprotected surface of the metal. The retention of moisture makes the corrosion continue, even when surfaces are dry.
The result is an acidic shift in pH, a concentration of Cl- ions and active corrosion. The negative ions of chlorine move under the deposit, balancing the positive ions of aluminum. This concentration of chloride ions brings about an acidic environment, which corrodes the metal.
It is quite easy to identify aluminum corrosion, as the product is white and more voluminous. It starts by the surface becoming rough, as well as etches and pits becoming more evident. The protective layer that forms shields the interior metal from corroding, but when removed, corrosion can become more vigorous.
Other areas that are prone to poultice corrosion are fuel tanks, due to placing and storing of cardboard and equipment, among other items, at the top of the tanks. Deck boxes found on aluminum sport boats are also prone to this type of corrosion. This is due to the storage of wet fishing nets, gear, life jackets and towels that can trap moisture on the aluminum's bare surfaces. The areas under chipped paint are another place that can trap moisture and initiate a corrosive process.
Maintenance of aluminum should be done with a pH of between 6 and 8, resulting in a stable alloy. When an acidic medium attacks metal, a basic solution should be applied to neutralize the effect.