What Does Bald Spot Mean?
A bald spot is a non-technical term for an area on a material's surface that has lost quality in its normal covering. The term can refer to a failure of a coating such as a wrinkle finish paint that has a rough texture. In the context of corrosion, bald spots refer to an area of thinned out steel material in a pipe under which cold-side corrosion has occurred.
Corrosionpedia Explains Bald Spot
Bald spots can form in the air port tubes found in recovery boilers. The underlying mechanism for the deterioration of the stainless steel is caused by cold-side corrosion. This type of corrosion occurs as a result of molten sodium hydroxide (NaOH) that was not neutralized from a previous reaction with carbon dioxide or sulfur oxides. The molten sodium hydroxide dissolves other corrosion byproducts such as sodium ferrite and chromic oxide, which provide protective barriers within the tubes. The unprotected surface is then prone to rapid corrosion. The corrosion thins the stainless steel, producing a bald spot. When noticed, the bald spots are fixed by welding overlays with stainless steel or other alloys.