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Common Problems and Preventive Measures of Stainless Steel Surface Treatment

2019-01-17 09:40

Stainless steel has unique strength, high wear resistance, superior anti-corrosion performance and not easy to rust and other excellent characteristics. Therefore, it is widely used in chemical industry, food machinery, mechanical and electrical industry, environmental protection industry, household appliances industry and household decoration, decoration industry, giving people a magnificent and noble feeling.

The application prospects of stainless steel will be wider and wider, but the application development of stainless steel largely determines the development degree of its surface treatment technology.

1. Common surface treatment methods for stainless steel

Introduction of 1.1 Stainless Steel Varieties

1.1.1 Stainless steel main components: generally contains chromium (Cr), nickel (Ni), molybdenum (Mo), titanium (Ti) and other high-quality metal elements.

1.1.2 Common stainless steel: chromium stainless steel, containing more than 12% of chromium; nickel-chromium stainless steel, containing 18% of chromium and 12% of nickel-chromium.

1.1.3 Classified from the metallographic structure of stainless steel: austenitic stainless steel, such as 1Cr18Ni9Ti, 1Cr18Ni11Nb, Cr18Mn8Ni5. Martensitic stainless steel, such as Cr17, Cr28, etc. Generally known as non-magnetic stainless steel and stainless steel with magnetic.

1.2 Common Surface Treatment of Stainless Steel

The common surface treatment technologies of stainless steel are as follows: 1) whitening treatment of the original color of the surface; 2) mirror brightening treatment of the surface; 3) surface coloring treatment.

1.2.1 Surface whitening treatment: During the process of stainless steel processing, black oxide skin is produced by rolling, edge binding, welding or artificial surface heating treatment. This kind of hard gray-black oxide skin is mainly composed of NiCr2O4 and NiF EO4, which were usually removed by strong corrosion with hydrofluoric acid and nitric acid before. But this method is expensive, polluting the environment, harmful to human body, corrosive and gradually eliminated. At present, there are two main methods to treat oxide skin:

(1) Sandblasting (pellet) method: mainly using the method of spraying glass beads to remove the black oxide skin on the surface.

(2) Chemical method: A non-polluting pickling passivating paste and a non-toxic cleaning solution with inorganic additives at room temperature were used for leaching. So as to achieve the purpose of whitening stainless steel. After treatment, it basically looks like a dull color. This method is more suitable for large and complex products.

1.2.2 Surface mirror polishing treatment of stainless steel: According to the complexity of stainless steel products and user requirements, mechanical polishing, chemical polishing, electrochemical polishing and other methods can be used to achieve mirror gloss. The advantages and disadvantages of these three methods are as follows:

1.2.3 Surface coloring treatment: Stainless Steel Coloring not only gives stainless steel products a variety of colors, increase the variety of products, but also improve the wear resistance and corrosion resistance of products.

There are several Stainless Steel Coloring methods as follows:

(1) Chemical oxidation coloring method;

(2) electrochemical oxidation coloring method;

(3) Ion deposition oxide coloring method;

(5) High temperature oxidation coloring method;

_Gas phase pyrolysis coloring method.

A brief overview of the various methods is as follows:

(1) Chemical oxidation coloring method: in a specific solution, the color of the film is formed by chemical oxidation, including dichromate method, mixed sodium salt method, sulfurization method, acid oxidation method and alkaline oxidation method. Generally, INCO is widely used, but if you want to ensure that a batch of products have the same color, you must use reference electrodes to control it.

(2) Electrochemical coloring: The color of the film is formed by electrochemical oxidation in a specific solution.

_Ion deposition oxides coloring chemical method: that is, stainless steel workpieces are placed in a vacuum coating machine for vacuum evaporation plating. For example: Titanium-plated watch case, watch band, generally golden. This method is suitable for mass product processing. Because of large investment and high cost, small batch products are not economical.

(4) High temperature oxidation coloring method: in a specific molten salt, the workpiece is immersed in a certain process parameters, so that the workpiece forms a certain thickness of oxide film, and presents a variety of different colors.

_Gas phase pyrolysis coloring method: more complex, less used in industry.

Selection of 1.3 Treatment Method

Which method to choose for surface treatment of stainless steel should be selected according to product structure, material quality and different requirements for surface.

2. Common causes of corrosion of stainless steel parts

2.1 Chemical Corrosion

2.1.1 Surface Pollution: Oil, dust, acid, alkali and salt attached to the surface of the workpiece are transformed into corrosive medium under certain conditions, and react with some components of stainless steel, resulting in chemical corrosion and rusting.

2.1.2 Surface scratches: The damage of various scratches to passivation film reduces the protective ability of stainless steel, and is easy to react with chemical media, resulting in chemical corrosion and rusting.

2.1.3 Cleaning: After pickling passivation, cleaning is not clean, resulting in residual liquid retention, direct corrosion of stainless steel (chemical corrosion).

2.2 electrochemical corrosion

2.2.1 Carbon steel pollution: scratches caused by contact with carbon steel parts and galvanic cells formed by corrosive medium lead to electrochemical corrosion.

2.2.2 Cutting: Slag cutting, splashing and other rusty substances adhere to the corrosive medium to form galvanic cells and produce electrochemical corrosion.

2.2.3 Baking Calibration: The composition and metallographic structure of the flame heating area changed unevenly, and formed galvanic cell with corrosive medium, resulting in electrochemical corrosion.

2.2.4 Welding: Physical defects (undercut, pore, crack, incomplete fusion, incomplete penetration, etc.) and chemical defects (coarse grains, poor chromium at grain boundaries, segregation, etc.) in the welding area form primary batteries with corrosive media