What is oxide skin?
What is oxide skin?
Oxide skin is the corrosion product of iron and steel caused by oxidation at high temperature. It consists of ferrous oxide, ferric oxide and ferric oxide. From the inside out: ferrous oxide, ferric oxide, ferric oxide. Among them, the structure of ferrous oxide is loose and the protective effect is weak, while the structure of Fe3O4 and Fe2O3 is dense and has better protective effect.
Generally, the measures to reduce metal oxidation are:
- (1) when the material temperature is below 1000 ℃, the oxidizing furnace gas can be used, so that the oxide skin formed can be easily removed, but at this time, the temperature is not high, and the oxidation is not fierce;
- (2) when the material temperature is above 1000 ℃, the reducing furnace gas is used, at this time, the amount of air entering the furnace should be reduced to avoid excessive oxide skin. In addition, rapid heating, short heating time, less oxidation and non oxidation heating are all measures to reduce metal oxidation.
Causes of oxide skin
When steel is heated, the chemical action of iron and oxygen or the product (CO2, H2O steam, etc.) produced when iron and fuel are burned forms an oxide layer, which is called oxide skin. Three layers are distributed on the surface of the heated billet; the inner layer is composed of fine particles of FeO, accounting for 40% of the total thickness of the oxide layer; the middle layer is composed of coarse particles of Fe3O4, accounting for about 50% of its thickness; the outer layer is composed of Fe2O3, accounting for about 10%. Because the expansion coefficient of iron oxide scale is different from that of steel, it is loose and easy to fall off. At the same time, the melting point of iron oxide scale (1300-1350 ℃) is low, so that when the steel is heated at high temperature, the falling off or melting of iron oxide scale will expose the metal surface, so that the oxidation will continue to accelerate.
Characteristics of oxide skin
The oxide cortex is brittle and has no extensibility. Under the mechanical and thermal processing effects, it is easy to crack and separate. Iron oxide and ferrous oxide form iron hydroxide under the action of water, which makes the oxide skin expand, crack and even fall off. In the original oxide skin, there are always cracks deep to the substrate. When the electrolyte rushes into the cracks, iron and oxide skin form the primary battery. The oxide skin is the cathode, and the iron acts as the anode to accelerate the corrosion. Therefore, the larger the oxide skin area is, the faster the corrosion rate of the steel matrix is, and the more serious the corrosion is.
Classification of scale
The scale can be divided into primary scale, secondary scale, tertiary scale and red scale.
Primary scale: before hot rolling, steel is usually heated and insulated at 1100-1300 ℃. At this temperature, the oxidation reaction of the steel surface in contact with the high temperature furnace gas results in the formation of 1 ~ 3mm thick primary scale, which is caused by insufficient side pressure and incomplete descaling during rough rolling. This primary scale is also called primary scale. There are large holes in the primary scale, and the primary scale is gray black scale, which is covered on the surface of the steel plate. The scale layer is mainly composed of magnet (Fe3O4).
Secondary oxide scale: after the hot rolling billet comes out of the heating furnace and the primary scale is removed by high pressure water, the oxide scale on the surface will fall off and rough rolling will be carried out. In a short period of rough rolling, the surface of billet contacts with water and air, and the surface of billet produces secondary scale, also known as primary scale. The thickness of secondary scale affected by horizontal rolling is thin, and the interface stress between billet and scale is small, so the peeling property is poor. If the secondary scale can not be completely removed by spraying high-pressure water and the scale remains on the surface of the steel plate, the surface of the product will be defective. The secondary oxide scale is a red scale layer, which is obviously long strip and pressed in shape, and distributed along the rolling direction. The scale layer is mainly composed of particles such as FeO and Fe2O3.
Three times iron oxide scale: in the process of hot rolling, the surface iron oxide scale will be produced when the strip enters each rolling mill. Oxide scale will be produced again after rolling through final descaling or between each rolling mill. Therefore, the surface condition of strip steel under the action of roll will depend on the quantity and characteristics of oxide scale formed before entering each rolling mill. At this time, the scale is called three times scale, because it is after descaling. Formed before entering the finishing mill.
The defects of three times iron oxide scale can be seen by naked eyes: dark brown, boat like. Relatively dense, fine and scattered sand distributed on the surface of the defective strip, with fine touch and hand feeling. After pickling, there are pinhole pits with different depths at the surface defects of the strip, which are invisible on the surface of normal hot rolled strip.
Red oxide scale: red oxide scale only occurs on specific steel grades with high silicon content, mainly due to the strong engagement of surface oxide and base metal during billet heating. There is no obvious depth and it is irregular.
There are two kinds of red iron oxide scale: one is distributed unevenly in the wide direction of the board, mainly in the middle, inclined to the operation side, with obvious watermarks in red and blue, uneven in the length direction of the board, and slightly lighter in some parts. This kind of red iron oxide scale is thick, which can collapse when straightening. It can be blown away by high-pressure wind. The residual red is easy to wipe off the color. This red iron oxide scale is called red rust.
Another kind of red iron oxide scale is evenly distributed along the width of the plate. Generally, it is heavier within 100mm of the edge and heavier outside than inside. This kind of red oxide sheet is thin and hard to wipe off the color. The thicker the steel plate, the heavier the red. This kind of red oxide scale also exists in some other steel grades, which has certain universality.
The harm of oxide skin
In addition to a large amount of steel loss, oxide skin will also cause many adverse consequences, mainly including:
The oxide scale roughens the forging surface. When forging, if the oxide skin is pressed into the forging, it will become a serious waste;
Some additional auxiliary processes and equipment are needed to remove oxide skin;
The oxide scale has high hardness, which not only increases the consumption of deformation energy, but also accelerates the wear of forging die and reduces the service life;
The oxide scale has the chemical corrosion effect on the refractory brick, which causes the furnace bottom to be damaged prematurely.
Removal method of oxide skin
Acid pickling with three acids (sulfuric acid + nitric acid + hydrochloric acid): acid pickling with three acids in a certain proportion is an effective acid pickling method to remove the oxide skin of stainless steel. Triacid pickling can remove the oxide skin of stainless steel smoothly, but the disadvantage is that there is a large amount of nitrogen dioxide precipitated in the pickling process, which worsens the working conditions and pollutes the environment. In addition, pickling process is easy to produce over pickling phenomenon, so it is not recommended to widely use this method to remove supply oxide skin.
Nitric acid + hydrofluoric acid pickling: this method is one of the better pickling methods for stainless steel. The commonly used ratio is as follows: HF 4% – 6%, HNO3 8% – 12% service temperature 50-600c pickling time depends on the oxide scale on the steel surface: generally 20-40 minutes. The operation process must be turned over frequently to prevent over pickling. The advantages of this pickling method are fast pickling speed, easy operation and good quality, but the disadvantages are: hydrofluoric acid is toxic and needs good ventilation conditions and wastewater treatment conditions.
The main purpose of alkaline cooking acid pickling complex method is to overcome the weakness of triacylic acid method. This composite method can not only improve pickling quality, but also reduce metal consumption. The scale can be loosened by alkali boiling and then removed by acid washing. The composition of alkaline cooking solution is generally a mixture of sodium hydroxide (NaOH) and sodium nitrate. The time of alkali boiling is related to the steel grade, specification, quantity and oxide skin state of the product. After alkali boiling, the steel should be immersed in the water tank immediately to remove the oxide skin, and then the oxide skin can be removed by acid washing. In particular, it should be noted that the alkali boiled steel should be dry and anhydrous to avoid splashing of hot alkali liquor. The operation process should be cautious and careful, and the surface changes must be observed and recorded more. To achieve the purpose of safe operation.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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