Which surface treatment process anti-corrosion effect is good?

2.2 Classification according to the thickness of the phosphating film

Phosphating film thickness (phosphating film weight), can be divided into four kinds of sub-lightweight, lightweight, sub-weight, heavyweight. Sub-lightweight film weight of only 0.1 ~ 1.0g/m2, generally amorphous iron phosphate film, only used for pre-paint priming, especially deformation of large workpieces before the paint priming effect is very good. Lightweight film weight 1.1 ~ 4.5 g/m2, widely used in pre-painting priming, in the anti-corrosion and cold processing industry applications less. Sub-weight phosphating film thickness of 4.6 ~ 7.5 g/m2, due to the film weight is larger, the film is thicker (generally & gt; 3μm), less as a pre-paint priming (only as a basic non-deformation of the steel parts of the pre-paint priming), can be used for corrosion prevention and cold machining friction lubrication. Heavyweight film is heavier than 7.5 g/m2, not as a pre-paint priming, widely used in corrosion prevention and cold processing.

2.3 Phosphate treatment temperature

Phosphate treatment temperature can be divided into room temperature, low temperature, medium temperature, high temperature four categories. Normal temperature phosphating is not heating phosphating. Low-temperature phosphating general processing temperature 30 ~ 45 ℃. Medium temperature phosphating general 60 ~ 70 ℃. High temperature phosphating is generally greater than 80 ℃. Temperature division method itself is not strict, sometimes there are sub-medium temperature, sub-high temperature method, depending on the will of each person, but generally follow the above division method.

2.4 Classification according to the type of accelerator

As the phosphating accelerator is mainly only so many kinds, according to the type of accelerator is conducive to the understanding of the tank. According to the type of accelerator can be determined by the phosphate treatment temperature, such as NO3-promoter is mainly medium-temperature phosphating. Promoter is mainly divided into: nitrate type, nitrite type, chloride type, organic nitride type, molybdate type and other major types. Each type of accelerator can be used in conjunction with other accelerators, there are many branches of the series. Nitrate type includes: NO3-type, NO3-/NO2-(autogenous type). Chlorate type includes: ClO3-, ClO3-/ NO3-, ClO3-/ NO2-. Nitrite type includes: Nitroguanidine R- NO2-/ ClO3-. Molybdate types include: MoO4-, MoO4-/ ClO3-, MoO4-/ NO3-.

There are many other ways to classify phosphating, such as according to the material can be divided into steel parts, aluminum parts, zinc parts and mixed parts phosphating.

Phosphating (Ⅱ) - pretreatment before phosphating

Generally, the phosphating treatment requires that the surface of the workpiece should be a clean metal surface (two-in-one, three-in-one, four-in-one exception). The workpiece must be pre-treated before phosphating in addition to grease, rust, oxide skin and surface adjustment. Especially before painting primed with phosphating also requires surface adjustment, so that the metal surface has a certain "activity", in order to obtain uniform, detailed, dense phosphate film, to improve paint adhesion and corrosion resistance requirements. Therefore, phosphating pretreatment is the basis for obtaining high-quality phosphating film.

1 Degreasing

Degreasing is aimed at removing grease and oil from the surface of the workpiece. Including mechanical and chemical methods. Mechanical method is mainly: manual scrubbing, sand blasting, flame burning. Chemical method: solvent cleaning, acidic cleaning agent cleaning, strong alkaline cleaning, low alkaline cleaning agent cleaning. The following is an introduction to the chemical method of grease removal process.

1.1 solvent cleaning

Solvent method in addition to grease, generally with non-flammable halogenated hydrocarbon vapor method or emulsification method. The most common is the use of trichloroethane, trichloroethylene, perchloroethylene vapor degreasing. Steam degreasing is fast, efficient, clean and thorough degreasing, and the removal of all types of oil and grease is very good. Add a certain amount of emulsion in chlorohalogen, no matter soaking or spraying effect is very good. Due to chlorinated halogen have a certain degree of toxicity, the vaporization temperature is also higher, and then due to the emergence of new water-based low alkaline cleaning agent, solvent vapor and emulsion degreasing method is now rarely used.

1.2 Acid cleaner cleaning

Acid cleaner degreasing is a very widely used method. It uses surfactant emulsification, wetting, penetration principle, and with the help of acid corrosion of metal to produce hydrogen mechanical stripping effect, to achieve the purpose of removing grease. Acid cleaners can be used at low and medium temperatures. Low temperature can only remove liquid oil, medium temperature can remove oil and grease, generally only suitable for immersion treatment. Acid cleaner mainly consists of surfactants (such as OP non-ionic active agent, anionic sodium sulfonate type), ordinary inorganic acid, corrosion inhibitor three major components. Because it has both rust and grease removal double function, people used to call it "two-in-one" treatment fluid.

Hydrochloric acid, sulfuric acid-based cleaning agent is the most widely used, low cost, high efficiency. But the acid cleaning residual Cl-, SO42- on the workpiece after corrosion is very harmful. And phosphate acid-based no corrosive residues of the hidden danger, but the cost of phosphoric acid is higher, the cleaning efficiency is lower.

For zinc parts, aluminum parts generally do not use acid cleaner cleaning, especially zinc parts in the acid corrosion is extremely fast.

1.3 strong lye cleaning

Strong lye in addition to grease is a traditional and effective method. It is the use of strong alkali saponification reaction of vegetable oil, the formation of saponification dissolved in water to achieve the purpose of removing grease. Pure strong lye can only be saponified to remove vegetable oils and fats and can not remove mineral oils and fats. Therefore, people add surfactants into strong lye, generally sulfonic acid anionic active agent, and use the emulsifying effect of surfactants to achieve the purpose of removing mineral oil. Strong lye in addition to the use of grease temperature are high, usually> 80 ℃. Commonly used strong alkali cleaning formula and process are as follows:

Sodium hydroxide 5% ~ 10%

Sodium silicate 2% ~ 8%

Sodium phosphate (or sodium carbonate) 1% ~ 10%

Surfactant (sulfonic acid) 2% ~ 5%

Processing Temperature > 80 ℃

Processing Time 5 ~ 20min

Treatment method: soaking, spraying can be

Strong lye degreasing requires high temperature, energy consumption, corrosive to the equipment, and the cost of materials is not low, so the application of this method is gradually reduced.

1.4 low alkaline cleaning fluid cleaning

Low alkaline cleaning fluid is currently the most widely used class of degreasing agent. It is low alkaline, general pH value of 9 ~ 12. less corrosion on the equipment, small damage to the surface of the workpiece state, can be used in low and medium temperature, in addition to the high efficiency of grease. Especially when used in spraying mode, the effect of grease removal is especially good. Low-alkaline cleaning agent mainly consists of inorganic low alkaline additives, surfactants, defoamers and so on. Inorganic additives are mainly sodium silicate, sodium tripolyphosphate, sodium phosphate, sodium carbonate and so on. Its role is to provide a certain alkalinity, with a dispersing suspension effect. It can prevent the grease taken off from being re-adsorbed on the surface of the workpiece. Surfactants are mainly non-ionic and anionic, generally polyvinyl chloride OP type and sulfonate type, which plays a major role in the process of removing grease. In special requirements also need to add some other additives, such as spraying the need to add defoamer, and sometimes also add surface adjusting agent, to play the degreasing, surface conditioning dual function. There are many commercialized low alkaline cleaners, such as PA30-IM, PA30-SM, FC-C4328, Pyroclean442 and so on.

Generally commonly used low alkaline cleaning solution formulations and processes are as follows:

Soak type Spray type

Sodium tripolyphosphate 4 ~ 10g / l 4 ~ 10g / l

Sodium silicate 0 ~ 10g / l 0 ~ 10g / l

Sodium carbonate 4 ~ 10g / l 4 ~ 10g / l

Antifoam 0 0.5 to 3.0g/l

Surface adjusting agent 0 ~ 3 g/l 0 ~ 3 g/l

Free alkalinity 5 ~ 20 points 5 ~ 15 points

Processing temperature room temperature ~ 80 ℃ 40 ~ 70 ℃

Processing time 5 ~ 20min 1.5 ~ 3.0min

Immersion cleaning agent should pay attention to the turbidity of surfactants. The main attention should be paid to the turbidity point of the surfactant, when the processing temperature is higher than the turbidity point, the surfactant precipitates and floats up, so that it loses the ability to degrease, and can be solved by adding anionic active agent in general. Spray-type cleaner should be added to sufficient defoamer, in the spray does not produce foam is particularly important.

Aluminum parts, zinc parts cleaning, must take into account their corrosion in alkaline conditions, it is generally appropriate to use a nearly neutral cleaning agent.

2 acid washing

Acid washing to remove rust and oxide skin is the most widely used method in the industrial field. The use of acid on the dissolution of oxides and corrosion of hydrogen generated by the mechanical stripping effect to achieve the purpose of removing rust and oxide skin. The most common acids used in pickling are hydrochloric acid, sulfuric acid and phosphoric acid. Nitric acid is seldom used due to the generation of toxic nitrogen dioxide gas in pickling. Hydrochloric acid pickling is suitable for use at low temperatures, should not exceed 45 ℃, the use of 10% to 45% concentration, should also be added to the appropriate amount of acid mist inhibitor is appropriate. Sulfuric acid pickling at low temperatures is very slow, suitable for use at medium temperatures, the temperature of 50 ~ 80 ℃, the use of 10% ~ 25% concentration. Phosphoric acid pickling has the advantage that it does not produce corrosive residues (hydrochloric acid, sulfuric acid pickling will be more or less less will be Cl-, SO42- residue), safer, but the disadvantage of phosphoric acid is that the cost is higher, the pickling speed is slower, the general use of the concentration of 10% to 40%, the processing temperature can be room temperature to 80 ℃. In the pickling process, the use of mixed acid is also a very effective method, such as hydrochloric acid - sulfuric acid mixed acid, phosphoric acid - citric acid mixed acid.

In the pickling descaling descaling tank liquid, must add the appropriate amount of corrosion inhibitor. Many types of corrosion inhibitors, selection is relatively easy, its role is to inhibit metal corrosion and prevent "hydrogen embrittlement". But pickling "hydrogen embrittlement" sensitive workpiece, corrosion inhibitor selection should be particularly careful, because some corrosion inhibitors inhibit two hydrogen atoms into hydrogen molecule reaction, that is: 2 [H] → H2 ↑, so that the concentration of hydrogen atoms on the surface of the metal to increase, enhance the "hydrogen embrittlement" Enhance the "hydrogen embrittlement" tendency. Therefore, it is necessary to consult the relevant corrosion data book, or do "hydrogen embrittlement" test, to avoid the selection of dangerous corrosion inhibitors.

>3 surface adjustment

The purpose of surface adjustment is to promote the formation of phosphating grain fine and dense phosphate film, as well as improve the speed of phosphating. There are two main types of surface adjusting agent, one is acidic surface adjusting agent, such as oxalic acid. The other is colloidal titanium. Both applications are very popular, the former also has the role of removing light rust (workpiece running process formed in the "water rust" and "wind rust"). In the phosphating pretreatment process, whether to choose the surface adjustment process and the choice of that kind of surface conditioning agent are determined by the process and the requirements of the phosphating film. The general principle is: priming phosphating before painting, rapid low-temperature phosphating requires surface conditioning. If the workpiece in the phosphating tank, has been the second rust, it is best to use acidic surface conditioning, but acidic surface conditioning is only suitable for ≥ 50 ℃ medium temperature phosphating. General medium temperature zinc and calcium phosphating without table conditioning is also OK. Phosphating pretreatment process is:

Degreasing - water washing - acid washing - water washing - Neutralization - surface conditioning - phosphating

Degreasing and descaling "two in one" - water washing --Neutralization--Surface Conditioning--Phosphatizing

Grease and Rust Removal--Water Cleaning --Surface conditioning - phosphating

Neutralization is generally 0.2% ~ 1.0% soda ash solution. In some processes for heavy grease workpiece, but also increase the pre-degreasing process.

Phosphating (Ⅲ) - phosphating process (1)