This entire corrosion thing is one of the many dark arts of boat maintenance faced by mariners today. Nothing really happens overnight – and in the case of corrosion related wastage of metals – the deterioration process is usually slow with ample but subtle tell tales along the way to give the alert mariner time to take corrective actions. One of the most insidious forms of corrosion is found as small patches or blooms of white powder-like deposits on aluminum. Poultice corrosion can happen on unprotected aluminum or under bubbling paint. In either case – the amount of wastage can be severe if not discovered and properly treated. Before looking at the cure – let’s discuss the cause.
Here’s a classic example of poultice corrosion – a bare aluminum window frame on a Hatteras which has been damaged by corrosion. Over a period of time, water trapped by debris and dirt against the aluminum frame formed an acidity material call aluminum hydroxide which has perforated the frame leaving white deposits scattered across the sill. Poultice or under deposit attack corrosion may occur when bare, unprotected aluminum surfaces are covered by absorbent materials, dirt, and debris trapping moisture against the unprotected metal surface. In this instance, corrosion attack can continue even when the other surfaces are dry due to the retention of moisture in the poultice. The corrosion mechanism is similar to crevice corrosion in that the covering materials or deposits act to limit the migration of oxygen to the covered area. This leads to acidic shifts in pH, concentration of Clˉ ions in the shielded area, and a more active corrosion potential under the deposit. Negatively charged chloride ions tend to migrate under the deposit to balance the positively charged aluminum ions produced there. The high concentration of chloride ions causes the area under the deposit to become more acidic compared to the bulk solution, further enhancing the corrosion under the deposit with a white, poultice-like material produced (aluminum hydroxide).
Corrosion attack on aluminum surfaces is usually quite obvious, since the products of corrosion are white and generally more voluminous than the original base metal. Even in its early stage, aluminum corrosion is evident as general etching, pitting, or roughness of the surface. Aluminum alloys commonly form a layer of smooth surface oxidation (0.001” to 0.025 thick). This is not considered detrimental as it provides a barrier against corrosion. But when this protective layer is removed – damage can and will happen.
Another real site for problematic poultice corrosion is aluminum fuel tanks. Don’t store cardboard boxes, equipment, or other items on top of tanks. Don’t use moisture wicking materials to cushion tanks against framing structures. ABYC recommends tankage to be installed in accordance with H-24 (gasoline fuel systems) which states that all non-integral tank supports, chocks, or hangers shall be separated from metallic tank surfaces by a non-metallic, non-moisture absorbent and non-abrasive material suitable for the purpose (e.g., neoprene, Teflon, and high density plastics) permanently bonded to the tank surface with impermeable, non-hydroscopic adhesive. Self-wicking material, such as carpet pile, shall not be in contact with a metallic tank.
Another common refuse for poultice corrosion attack is in the deck boxes of aluminum sport boats where stowed fishing gear, nets, towels, and life jacket can trap moisture against bare aluminum.
Poultice corrosion can also occur under chipped paint – the mechanism is the same – trapped moisture against unprotected aluminum. Look at paint bubbles – such as around beauty rings on portholes (don’t be confused by galvanic corrosion). Chipped paint or coatings will promote poultice corrosion – allowing moisture to be trapped against bare metal. The best form of prevention is to properly coat all bare metal surfaces. If not possible, bare aluminum must be kept clean and dry – free of wicking materials. Think about inside airplane wings – aluminium is always coated to prevent corrosion. Maintain regular cleaning and proper coating of all exposed surfaces to prevent corrosion.
Unprotected aluminium should be maintained within a pH range of 6 to 8 where the alloy is stable. In the event of poultice attack, a base solution should be applied to control acidic corrosion. Protective steps would include, but not limited to the use of Alodine®. This material is one step in a multi-stage protection scheme and does not provide sacrificial protection to aluminum alloys. Instead, it acts as a passivating inorganic thin coating over which a primer can be applied. In the same manner, a zinc chromate primer does not provide sacrificial protection but its corrosion inhibiting properties retard but does not prevent corrosion.