Stainless steel surface treatment process

 Stainless steel has unique strength and superior anti-corrosion properties. At the same time, it has high wear resistance and is not easy to rust. Therefore, it is widely used in the chemical industry, food machinery, electromechanical industry, and household appliance industry. The application and development prospects of stainless steel will become wider and wider, but the application and development of stainless steel largely determine the degree of development of its surface treatment technology.

Introduction of stainless steel varieties

Stainless steel generally contains high-quality metal elements such as chromium (CR), nickel (NI), molybdenum (MO), and titanium (TI). Common stainless steels include chromium stainless steel and nickel-chromium stainless steel. Classification from the metallographic structure of stainless steel: austenitic stainless steel, martensitic stainless steel

Stainless steel surface treatment varieties

• Whitening treatment of the natural color of the surface
• Surface mirror stone bright treatment
• Surface coloring treatment

1. The surface natural color whitening treatment

In the process of processing, the stainless steel undergoes coiling, edge binding, welding, or the handler’s warm surface fire treatment, resulting in a black oxide scale. This kind of hard gray-black oxide scale was usually removed by a strong corrosion method with hydrofluoric acid and nitric acid in the past. However, this method is costly, pollutes the environment, and is harmful to the human body. The corrosion is relatively large, and it is gradually eliminated.

At present, there are two kinds of treatment methods for this kind of oxide scale: (1) Using shot peening/sandblasting method. (2) Use chemical methods.

2. Mirror bright treatment method of stainless steel surface

According to the complexity of stainless steel products and the different requirements of users, mechanical polishing, chemical polishing, electrochemical polishing, and other methods can be used to achieve mirror stone luster.

Mechanical polishing

Good leveling performance, bright labor intensity, serious pollution, difficult to process complex parts, reduced gloss, high investment, and cost. Simple workpieces, medium and small products, and complex parts cannot be processed. The gloss of the entire product is not consistent, and the gloss retention time is not long.

Chemical polishing

Low investment, complex parts can be polished, high efficiency, high speed, insufficient brightness, polishing liquid to be humidified, gas overflows, and wind-friendly equipment is required.

Electrochemical polishing

Mirror gloss, long-term maintenance, stable process, less pollution, low cost, and good anti-pollution performance. The one-time investment is large, and the complicated parts must be equipped with tooling and auxiliary electrodes.

3. Surface coloring treatment

Stainless steel coloring not only imparts various colors to stainless steel products, increases the variety of products, but also improves the wear resistance of products.

There are several coloring methods for stainless steel:

(1) Ion deposition of oxides or oxides

(2) High-temperature oxidation method

(3) Chemical oxidation method, electrochemical oxidation method

(4) Gas-phase cracking method

Briefly describe the overview of various methods:

(1) The ion deposition oxide or oxide method is to place the stainless steel workpiece in a vacuum coating machine for vacuum evaporation plating; this method is suitable for mass product processing. Because of the large investment and high cost, small-batch products are not economical.

(2) The high-temperature oxidation method is to immerse the workpiece in a specific molten salt to maintain a certain process range, so that the workpiece forms a certain thickness of oxide film, and presents a variety of different colors.

(3) The chemical method is to form the color of the film through chemical oxidation in a specific solution. However, if the color of a batch of products is consistent, it must be controlled by a reference telegram.

(4) The electrochemical method is to form the color of the film through electrochemical oxidation in a specific solution.

(5) The gas-phase cracking method is more complicated and less used in industry.

Electroplating surface treatment methods

Electroplating is a method in which metal is precipitated from the electrolyte by the action of an applied current and deposited on the surface of the object to obtain a metal covering layer.

Galvanized:

Zinc is easily corroded in acids, alkalis, and sulfides. The zinc layer is generally passivated. After passivation in chromate solution, the passivation film formed is not easy to interact with moist air, and the anti-corrosion ability is greatly enhanced. In dry air, zinc is relatively stable and not easy to change color. In water and humid atmosphere, it reacts with oxygen or carbon dioxide to form an oxide or alkaline carbonic acid film, which can prevent zinc from continuing to oxidize and play a protective role.

Applicable materials: steel, iron parts

chrome:

Chromium is very stable in a humid atmosphere, alkali, nitric acid, sulfide, carbonate solutions, and organic acids, and is easily soluble in hydrochloric acid and hot concentrated sulfuric acid. The disadvantage is that it is hard, brittle, and easy to fall off. Direct chromium plating on the surface of steel parts as an anti-corrosion layer is not ideal. Generally, multi-layer electroplating (ie copper plating → nickel → chromium) can achieve the purpose of rust prevention and decoration. At present, it is widely used to improve the wear resistance of parts, repair size, light reflection, and decoration.

Applicable materials: ferrous metal, copper, and copper alloy zero decorative chrome plating, wear-resistant chrome plating

Copper plating:

Copper is not stable in the air, and at the same time, it has a high positive potential and cannot protect other metals from corrosion. However, copper has high electrical conductivity, the copper plating layer is tight and fine, it is firmly combined with the basic metal, and it has good polishing performance. It is generally used to improve the conductivity of other materials, as the bottom layer of another electroplating, as a protective layer to prevent carburization, and to reduce friction or decoration on the bearing.

Applicable materials: black metal, copper, and copper alloy nickel-plated, the chrome-plated bottom layer

Nickel plating:

Nickel has good chemical stability in the atmosphere and lye and is not easy to change color, but it is easily soluble in dilute nitric acid. It is easy to passivate in concentrated nitric acid, and its disadvantage is porosity. To overcome this disadvantage, multi-layer metal plating can be used, and nickel is the intermediate layer. The nickel plating layer has high hardness, is easy to polish, has high light reflectivity and can increase the appearance and resistance, and has good corrosion resistance.

Applicable materials: can be deposited on the surface of various materials, such as steel-nickel-based alloys, zinc-based alloys, aluminum alloys, glass, ceramics, plastics, semiconductors, and other materials

Tin plating:

Tin has high chemical stability. It is not easy to dissolve in dilute solutions of sulfuric acid, nitric acid, and hydrochloric acid. Sulfides have no effect on the tin. Tin is also stable in organic acids, and its compounds are non-toxic. It is widely used in food industry containers and parts of aviation, navigation, and radio equipment. It can be used to prevent copper wires from being affected by sulfur in rubber and as a protective layer for non-nitriding surfaces.

Applicable materials: iron, copper, aluminum, and their respective alloys

Copper tin alloy:

Copper-tin alloy electroplating is to plate copper-tin alloy on the parts without nickel plating, but directly chromium plating. Nickel is a relatively rare and precious metal. At present, copper-tin alloy electroplating is widely used in the electroplating industry to replace nickel plating, which has good anti-corrosion ability.

Applicable materials: steel parts, copper, and copper alloy parts