When dirt and other debris accumulate in certain metallic areas, a unique highly corrosive environment can be created. Example areas on automobiles include the hem flange within doors, wheel wells, and inside frames. These areas remain wet almost continuously with a highly corrosive liquid due to the moisture entrapment effect of the poultice. The aggravation caused by deicing salts can be quite serious in these areas due to wet dry cycling and accumulation that can reach saturation. The stationary electrolyte can become increasingly acidic in this particular form of crevice corrosion. A detailed analysis of poultices collected on fifty cars driven in four major North American cities has revealed the presence of large quantities of ions such as sodium, calcium, sulfate and chloride.
Average composition of poultice (ppm) found in fifty cars
In these cities, sodium, calcium, and chloride deposit in poultices through road deicing and dust control practices. Sulfate can be attributed to acid deposition commonly associated with pollution. (reference)