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The surface treatment technology of stainless steel
Feb 02, 2017

Stainless steel has a unique strength, high wear resistance, excellent corrosion resistance and rust and other excellent features. It is widely used in chemical industry, food machinery, electrical and mechanical industry, environmental industry, household appliances and home decorating industry, finishing industry, giving people a beautiful and noble feeling.

 

Prospects for the development of stainless steel will be more widely applied, but the development of application of stainless steel surface treatment largely determine its level of technological development.


stainless steel surface treatment methods commonly used varieties

 

1.1 Introduction 1.1.1 stainless steel main components: general contain chromium (Cr), Nickel (Ni), molybdenum (Mo), titanium (Ti) and other high-quality metals.

1.1.2 Common steel: stainless steel with chromium, including Cr ≥ 12% or more; nickel-chromium stainless steel with Cr ≥ 18%, with Ni ≥ 12%.

1.1.3 Microstructure of structure from stainless steel: a austenitic stainless steel, for example: 1Cr18Ni9Ti, 1Cr18Ni11Nb, Cr18Mn8Ni5. Martensitic stainless steel, for example: Cr17, Cr28 and so on. Commonly referred to as non-magnetic stainless steel and with a magnetic stainless steel.

 

1.2 Common stainless steel surface treatment methods commonly used stainless steel surface treatment technology has the following approach:

① surface color bleaching treatment;

② mirror-bright surface treatment;

③ surface shading.

 

1.2.1 Surface color bleaching treatment: stainless steel in the process, after the coil, bar-side, welding or heat treatment after the artificial surface of the roast, producing a black oxide. This hard gray-black oxide is mainly NiCr2O4 and NiF two kinds EO4 components, before the general use of hydrofluoric acid and nitric acid to remove corrosion strong. However, the cost of this approach large, polluting the environment, harmful, corrosive larger, gradually be eliminated. Oxide present there are two treatment methods:

⑴ sandblasting (pill) method: The main jet is the use of micro-glass beads method, remove the black oxide surface.

⑵ chemical method: the use of a non-polluting pickling and passivation paste at room temperature with a non-toxic inorganic additives, cleaning fluid immersion. Qualities of stainless steel so as to achieve the purpose of whitening. Basically looks after the deal is a light color. This method for large, complex products are more applicable.

 

1.2.2 mirror-bright stainless steel surface treatment: stainless steel products according to user requirements and the complexity of the situation is different, respectively mechanical polishing, chemical polishing, electrochemical polishing and other methods to achieve the glossy shine.


Advantages and disadvantages of these three methods are as follows:

1.2.3 Surface shading: stainless steel stainless steel color not only gives a variety of colors, increase product varieties and improve product wear and corrosion resistance. There are several methods of stainless steel color:

⑴ chemical oxidation coloring method;

⑵ electrochemical oxidation coloring method;

⑶ coloring oxide ion deposition method;

⑷ high-temperature oxidation coloring method;

⑸ gas cracker coloring.

 

A brief overview of various methods are as follows:

⑴ chemical oxidation coloring: that a particular solution, formed by chemical oxidation of the color film, there dichromate method, mixed sodium method, curing method, acid oxidation and alkaline oxidation . General "Yin Division Act" (INCO) use more, but a number of products in order to ensure consistent color, must be controlled with the reference electrode.

⑵ electrochemical coloring method: in a specific solution, formed by electrochemical oxidation of the color film.

⑶ ion deposition oxide coloring chemical method: the workpiece is placed in a stainless steel vacuum coating machine by vacuum evaporation coating. For example: titanium gold watch cases, watch band, usually gold. This method is suitable for high-volume processing. Because the large investment, high cost, low-volume products uneconomical.

⑷ oxidation coloring: in particular molten salt, the immersion of the workpiece to maintain a certain process parameters, so that the workpiece to form a certain thickness of oxide film, while showing a different color.

⑸ gas cracking shading: more complex, seldom used in industry.

 

1.3 selection of stainless steel surface treatment method of choice which, according to product structure, material, and different requirements on the surface, use the appropriate method of treatment.


2. stainless steel parts from corrosion of the common causes of corrosion

2.1.1

2.1 Surface chemical contamination: attached to the surface of the oil, dust and acid, alkali and salt corrosion under certain conditions, into the medium, and stainless steel parts in some of the chemical composition reaction, chemical corrosion and rust.

2.1.2 scratch the surface: a variety of scratch damage of the passive film, the lower stainless steel protection, easy to react with the chemical mediators, chemical corrosion and rust.

2.1.3 Cleaning: Cleaning is not clean after pickling passivation caused by residue retention, direct corrosion stainless steel parts (chemical attack).

 

Electrochemical corrosion of carbon steel contamination

2.2

2.2.1: carbon steel with scratch and corrosion caused by exposure to medium to form the original cells resulting electrochemical corrosion.

2.2.2 Cutting: cutting slag, splatter and other material attached to rust and corrosion medium to form the original cells generated electrochemical corrosion.

2.2.3 grilled School: flame heating the composition and microstructure of the regional changes and uneven, and the corrosive medium to form the original cells arising from electrochemical corrosion.

2.2.4 Welding: welding area of physical defects (undercuts, pores, cracks, lack of fusion, incomplete penetration, etc.) and chemical defects (coarse grains, grain boundary chromium depletion, segregation, etc.) and corrosive medium to form the original cells and produce electrochemical corrosion.

2.2.5 Material: stainless steel chemical defects (non-uniform composition, S, P impurities, etc.) and physical surface defects (loose, sand holes, cracks, etc.) will help with the corrosive medium to form the original cells arising from electrochemical corrosion.

2.2.6 Passivation: stainless steel pickling passivation effect is caused by poor or uneven surface passive film is thin, easy to form the electrochemical corrosion.

2.2.7 Cleaning: retention of stainless steel pickling passivation chemical residue and the resultant corrosion of stainless steel parts with the formation of electrochemical corrosion.


2.3

The stress concentration caused by stress corrosion easy to short, stainless steel due to its special microstructure and surface passivation film, making it more difficult under normal circumstances, a chemical reaction with the media corrosion, but not in any condition not to be corrosion. In corrosive media and incentives (such as scratches, splashes, cutting slag, etc.) under the conditions of existence, corrosive stainless steel can occur with slow chemical and electrochemical corrosion reactions and corrosion under certain conditions, very fast and produce corrosion phenomena, in particular, pitting and crevice corrosion. The corrosion mechanism of stainless steel parts are mainly electrochemical corrosion. Thus, in the process of stainless steel products in the processing operations should take all effective measures to prevent corrosion conditions and incentives for production. In fact, many corrosion conditions and incentives (such as scratches, splashes, cutting slag, etc.) for the appearance of the product quality has a significant adverse impact, it should and must be overcome.

 

3. Stainless steel products processing problems

 

3.1 defects in welds: weld defect is more serious, the use of hand-processing methods to compensate for mechanical polishing to produce the grinding marks, resulting in uneven surface and affect appearance.

3.2 Surface inconsistent: only weld pickling passivation, but also result in uneven surface and affect appearance.

 

Difficult to remove scratches.

3.3: Overall pickling passivation, nor can the processing for a variety of scratches removed, and can not be removed as scratches, weld splatter and adhere to the stainless steel surface, splash, etc. impurities, resulting in corrosive media under the conditions of the existence of a chemical or electrochemical corrosion and rust corrosion.

3.4 Passivation polishing uneven: hand-polished after pickling passivation of the larger piece, it is difficult to achieve uniform treatment effect, can not get the desired uniform surface. And work expenses, supplies and higher costs.

3.5 pickling capacity is limited: pickling and passivation paste is not a panacea for plasma cutting, flame cutting and production, and black oxide, is more difficult to remove.

3.6 scratches caused by human factors more serious: the lifting, transport and structural processing, bump, drag, hammering scratches caused by human factors such as more serious, making the surface more difficult, but also dealing with the resulting corrosion the main reason.

3.7 Equipment factors: the profile, plate roll bending, bending process, resulting in scratches and creases are also the main reason for rust treatment.

3.8 Other factors: the stainless steel raw materials in the procurement, storage process, due to lifting, transport processes of the bump and scratch but also more serious, is one of the causes rust.


4. Should take preventive measures

4.1 storage, lifting, transportation

4.1.1 stainless steel parts store: there should be special storage racks, storage racks should be wood or steel frame painted surface with rubber mats or pads, in line with carbon steel and other metal materials isolation. Storage, the storage location should be easy lifting, and other materials storage area is relatively isolated, there should be safeguards to prevent dust, oil, rust on stainless steel contamination.

4.1.2 Stainless steel lifting components: lifting, should be dedicated spreader, such as lifting belts, special chucks, etc., prohibited the use of steel wire rope so as not to scratch the surface; and in the lifting and placement should avoid scratches caused by the impact of the bump.

4.1.3 Stainless steel pieces of transportation: transportation, transportation applications (such as car, car batteries, etc.), and isolation precautions have to be clean to prevent dirt, oil, pollution, rust stainless steel.

 

Non-drag, to avoid the bump, scratch.

4.2 Processing

4.2.1 Processing Zones: Stainless steel parts processing area should be relatively fixed. Stainless steel parts processing zone isolation measures should be taken to the platform, such as covered with rubber mats. Stainless steel parts processing zones set management, civilized production should be strengthened to avoid damage to the stainless steel parts and pollution. 4.2.2 Cutting: Cutting stainless steel parts using cutting or plasma cutting, sawing, etc.

⑴ cut: cut, the stent should be sent to isolation, drop hopper should be paved with rubber pads to prevent scratches.

⑵ plasma cutting: Plasma cutting, the cutting residue should be cleaned. Batch cutting for the completed parts should be promptly cleared the scene in order to avoid cutting the stain residue on the workpiece.

⑶ sawing Cutting: When cutting cutting, clamping rubber should be protected, clean the workpiece after cutting oils, residues, etc.

4.2.3 machining: stainless steel parts in the car, milling and other machining should also pay attention to protection, work to complete the surface should be cleaned of oil, iron and other debris.

4.2.4 Forming: in coil, bending process, should take effective measures to avoid surface scratches and creases stainless steel.

4.2.5 riveting: stainless steel pieces in the group should avoid forcing assembly, in particular, to avoid flame grilled school assembly. Group or production process where the temporary use of plasma cutting, isolation measures should be taken to avoid cutting stainless steel slag to other parts of the pollution. After cutting, the workpiece on the cutting residue should be cleaned.

4.2.6 Welding: welding stainless steel must be carefully removed before the oil, rust, dust and other debris. TIG welding as far as possible, using the manual metal arc welding should be used for small current, welding speed, to avoid the swing. Prohibited in the area of non-arc welding, ground location suitable to connect securely to avoid scratches arc. Measures should be taken to prevent weld spatter (lime and other methods such as brush). Applied after welding stainless steel (not carbon steel) flat shovel and a thorough clean-up slag splashing.

4.2.7 multi-layer welding: Multi-welding, the slag layer must be removed. Multi-layer welding, interpass temperature should be controlled, generally no more than 60 .

4.2.8 Weld: Weld joints should be grinding, weld surface may not have slag, porosity, undercut, spatter, cracks, lack of fusion, incomplete penetration defects, weld and base metal should be smooth transition, not low in the base metal.

4.2.9 orthopedic: orthopedic stainless steel parts, should avoid the use of flame heating method, in particular, repeated heating does not allow the same area. Orthopedic, try to use mechanical devices, or wooden hammer (rubber hammer) or a rubber mat mat hammer, ban hammer with a hammer to avoid damage to stainless steel.

4.2.10 Handling: stainless steel components for handling during processing, the application of means of transport (such as cars, electric vehicles or cranes, etc.), and isolation precautions have to be clean to prevent dirt, oil, pollution, rust stainless steel.

 

Prohibited direct drag on the platform or ground, is strictly prohibited bump and scratch.

4.3

4.3.1 clean polished surface: if any damage should be polished, in particular, caused by contact with carbon steel scratches and splash, the damage caused by cutting slag must be carefully and thoroughly clean polished clean.

4.3.2 mechanical polishing: polishing to the use of appropriate tools for polishing, requires uniform treatment, and to protect against scratches and re-cast.

4.3.3 degreasing dust: stainless steel pickling passivation before making pieces, must craft clear oil, oxide, dust and other debris.

4.3.4 Water blasting: according to different processing requirements, use of different micro-glass beads, different process parameters, and to avoid overspray, etc.

4.3.5 Pickling Passivation: stainless steel pickling passivation parts must be in strict accordance with technical requirements for passivation.

4.3.6 Dry cleaning: pickling passivation, should be carried out in strict accordance with technical and, rinsing and drying to remove residual acid.

4.3.7 Protection: Stainless steel surface treatment has been completed, should be good protection, avoid touching people and oil, dust and other debris of secondary pollution.

4.3.8 to avoid re-processing: After the stainless steel surface treatment, avoid the parts or products of reprocessing. Yunqing brand stainless steel pickling additive uses: in stainless steel pickling process of adding nitric acid (HNO3) or sulfuric acid (H2SO4) in line with its use, to promote a variety of stainless steel black oxide, silicate removal of scale, reduce nitric acid or sulfuric acid corrosion of the metal matrix, while inhibiting the production of acid mist.

 

In normal use to greatly reduce the corrosion rate of stainless steel, and excellent rejection of stainless steel in the pickling process of hydrogen absorption ability to avoid the occurrence of stainless steel "hydrogen embrittlement", while inhibition of Fe3 + in the process of pickling of metal corrosion, the stainless steel pitting phenomenon does not occur. For various types of stainless steel. Features: powdery solid, with the use of low concentration, the effect is good, stable performance, simple operation, the amount of small, high efficiency, low cost; the corrosion of the metal matrix is small, corrosion rate, the process does not mist, use safety. Usage: pickling: acid - washing - Shot Blasting - acid - wash - and in - washing - using hot water additive concentration is generally 5-10% (weight), the measurement of the additive metering water, stirring dissolved can. Can be added nitric acid or sulfuric acid, add additives, can be additive added nitric acid or sulfuric acid; room temperature or heated to 50 to 60 degrees to use, will have to deal with the stainless steel pieces soaked in cleaning solution, for 5 to 30 minutes or more time (processing temperature and time by the oxide thickness, plate material and processing requirements), to the surface oxide, rust completely cleaned, so far as silver, then with water (lime or alkaline better) rinse avoid back rust. Pickling temperature 48 degrees: 1 ton of water: 150L nitric acid: 30 kg additives pickling pickling pickling temperature 60 minutes at room temperature: 1 ton of water :200-300L nitric acid: 60 kg pickling additives pickling time is 40 minutes 1 tons of acid additives can handle 600 tons stainless steel (the data for reference only).