In hot-rolled steel production and welding processes, iron oxide scale is the most common surface product. It accompanies the entire process of steel production, from heating and rough rolling to finish rolling, directly affecting the appearance quality of the steel and the subsequent welding results. Many welding professionals deal with it daily, yet may not be able to distinguish the causes, forms, and hazards of different types of iron oxide scale. Based on the formation stage and material characteristics, steel surface iron oxide scale is mainly divided into four categories: primary iron oxide scale, secondary iron oxide scale, tertiary iron oxide scale, and red iron oxide scale. These will be explained in detail below.
Primary Iron Oxide Scale: Before hot rolling, steel is often heated and held at 1100℃~1300℃. At this temperature, the steel surface undergoes an oxidation reaction upon contact with the high-temperature furnace atmosphere, generating a 1~3mm thick primary scale, which is also caused by insufficient side pressure during rough rolling and incomplete descaling. This primary scale is also called primary iron oxide scale. Primary scale contains large cavities and is a grayish-black scale layer, covering the steel plate surface in flake form. The scale layer is mainly composed of magnetite (Fe3O4).
Secondary iron oxide scale: After hot-rolled steel billets exit the furnace, they undergo high-pressure water treatment to remove primary scale, causing the surface iron oxide scale to detach before rough rolling. During the short rough rolling process, the billet surface comes into contact with water and air, resulting in secondary scale, also known as primary iron oxide scale. Due to the horizontal rolling process, the secondary scale is thinner, and the interfacial stress between the billet and the scale is low, leading to poor peeling properties. If the high-pressure water jet cannot completely remove the secondary scale, leaving residue on the steel plate surface during finish rolling, surface defects will appear on the product. Secondary iron oxide scale is a reddish layer, appearing as distinct long strips, indented, and distributed in bands along the rolling direction. The scale layer is mainly composed of particles such as ferroalloy (FeO) and hematite (Fe2O3).
Tertiary iron oxide scale: During the hot-rolled finish rolling process, a surface iron oxide scale layer is generated as the strip enters each mill stand. After rolling, iron oxide scale will be generated again during final descaling or between mill stands. Therefore, the surface conditions of the strip under the action of the rolls will depend on the quantity and characteristics of the iron oxide scale formed before entering each stand of the rolling mill. This iron oxide scale is called tertiary iron oxide scale because it is formed after descaling and before entering the finishing mill. Its defects are visible to the naked eye: dark brown, boat-shaped. It is relatively dense, fine, and scattered on the surface of the defective strip, and can be felt by hand. After pickling, it leaves pinhole-like pits of varying depths at the defective areas of the strip surface; these are not visible on the surface of normal hot-rolled strip.
Red iron oxide scale: Red iron oxide scale only occurs on specific steel grades such as those with high silicon content, mainly due to the strong interlocking of surface oxides with the base metal during the billet heating process. It has no obvious depth and appears as irregular flakes. Specifically, it is divided into two types.
One type is non-uniformly distributed in the width direction of the plate, mainly distributed in the middle, biased towards the operating side. There are obvious watermarks in the red and blue areas. It is also uneven in the length direction of the steel plate, with some areas being slightly lighter. This first type of red iron oxide scale is relatively thick and can flake off during straightening. It can be blown away with high-pressure air, and the residual red color is easily wiped off. Calling this type of red iron oxide scale "red rust" is more accurate.
Another type of red iron oxide scale is more evenly distributed along the width of the plate, generally slightly heavier within the first 100mm of the edge, and heavier on the outside of the coil than the inside. This type of red iron oxide scale is thinner and not easily wiped off; the thicker the steel plate, the heavier the red color. This type of red iron oxide scale also exists in some other steel grades and is quite common.
