What causes steel to rust in thick flakes, and how can it be treated?
Hot rolled steel plate is produced by repeatedly rolling a cast steel ingot down to the finished thickness. This has the effect of creating a grain in the sheet longitudinal to the direction of roll, it also homogenises impurities such as dissolved gases, voids and inclusions present in the cast steel ingot. This process greatly strengthens the material but creates a stratified microstructure. When the steel corrodes this structure causes oxidisation to propagate along the grain boundaries known as Lamellar Corrosion or more commonly ‘pancake rusting’. Iron oxide can expand between 7 and 11 times its original thickness and as the oxide expands it forces the grain structure apart into layers of corrosion.
Corrosion inhibiting primers and rust treatments cannot penetrate through the layers of pancake rust to work on any corrosion active in the underlying layers. It is essential therefore to remove any flaking layers of pancake rusting until white metal is found before treatment. This oxide is very hard and brittle and will not react well to abrasion with sand paper or wire brushes; these will just polish the surface. The surface may look clean but unless it appears as white metal, the rust will not have been removed. Its removal is best achieved with a needle gun or by grit blasting until clean metal is seen.
It is important to discover whether the corrosion appears uniform across the surface or as a deep groove along the line of the weld. This groove corrosion is a form of hydrogen embrittlement and is more serious as it can significantly affect the strength of the weld. If a groove or crack is found in the surface of the cleaned steel along the line of, or parallel to the line of the weld, it must be ground out and the seam re-welded. It is important to use low hydrogen welding rods, known as ‘Low-hy’ rods to limit any absorption of hydrogen into the steel. Mig welding is also recommended as it naturally a low hydrogen weld process.