Bare spots are a steel defect that occur because of surface contamination. Sometimes called pits, pocks or blemishes this defect occurs as a result of atmosphere problems in the line furnace, excessive temperature of the steel entering the galvanizing bath, slag bounce, rolling defects or inadequate surface preparation. Sometimes these bare spots can be small enough to be labeled as “self-healing” however uncoated or irreparable spots are almost certainly grounds for rejection.
For castings, sand embedded in castings are the # 1 cause of bare spots. Excess aluminum in the galvanizing kettle and even lifting devices such as forks or grasper points that prevent coating formation in a small area. To avoid bare spots the galvanizer must ensure surfaces are clean and rust-free after pretreatment. In the case of a rejection the parts may be stripped and regalvanized. This can be a costly mistake for a master coil and may be unworthy of a regalv.
If you really want to geek out about bare spots – check out this article on Scientific.net.
Here is a brief excerpt from the Materials Science Forum article:
“Bare spots defect of galvanealed (GA) in High Strength Dual Phase (DP) steel strip with 1.5 % Mn contain was studied in detail. The surface morphologies of spot defects before and after partial and complete removal of the Zn layer, as well as the interface between the outermost coating layer and the sheet substrate were analyzed by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray Photoelectron spectroscopy (XPS), glow discharge optical emission spectrometry (GDOES) and laser scanning confocal microscope(LSCM). It was found that the bare spots defect was composed of a large number of pits with different sizes and depths. There were a lot of Fe – Zn alloy particles distributed in the pits, and many MnO formed on the surface, no effective Fe2Al5 inhibition layer formed. The results showed that the main reasons for the bare spots defect of the GA in High Strength DP steel strip are as follows: a silicon oxide film forms on the substrate during annealing prior to hot dipping because of Mn gathered on the surface of steel strip, thus strip surface wettability with liquid zinc is deteriorated and preventing the formation of the Fe2Al5 inhibition layer during hot dipping. In this paper, the dew point control process is introduced creatively, by increasing the dew point and the hydrogen content in the furnace area, and the bare spot defects on the surface of the high strength duplex steel galvanized sheet are solved. “