Q:What causes pits on the surface of hot-dip galvanized steel pipes?

The steel pipe billet is severely corroded, and a thick iron oxide layer appears on the surface. Because the elements in the steel matrix are not necessarily uniform, some places are deeply corroded, and some places are shallowly corroded. After pickling, large uneven pits appear. Such a surface leaves pits after heat leveling.

Time：08/01/2025 16:57:54

The steel pipe billet is too corroded, and a thick iron oxide layer appears on the surface. Because the elements in the steel matrix are not necessarily uniform, some places are deeply corroded, and some places are shallowly corroded. After pickling, large uneven pits appear. Such a surface still leaves pits after hot-dip galvanizing.

When the steel pipe is pickled, many pits are caused by over-pickling, especially in the welds. The action of micro-batteries causes this. Too large pits cannot be filled during hot-dip galvanizing.

The carbon (C) content, iron carbide (Fe3C), silicon, aluminum, copper, phosphorus, sulfur, and nitrogen in the steel pipe matrix will cause different dissolution rates during pickling and cause pits. This situation is very easy to occur in basic converter steel, so it is also easy to cause over-pickling.

The pickling steam scars formed during pickling are ring-shaped or bird's nest-shaped, which are easy to cause pits.

Time：07/01/2025 13:02:28