Introduction of degreasing and phosphating process before steel drum coating

Introduction of degreasing and phosphating process before steel drum coating

Hu Bing (Chengdu Yinguang Surface Technology Co., Ltd.)

Chengdu Yinguang Surface Technology Co., Ltd. was established in 1998 and has been engaged in the industry for 11 years. Our company specializes in the production and processing of surface treatment agents before development; our company is a member of Chengdu Mechanical Engineering Society, ISO9001-2000 international quality system certification enterprise, is a member of Sichuan Product Quality Inspection Center, and the product is Sichuan Quality and Technical Supervision Center. In 2006, the Sichuan market quality tracking qualified products were rated as the 2006 “Quality, Law, and Integrity” quality credit alliance enterprise by Sichuan Quality and Technical Supervision Center.

Our company has offices in Urumqi, Zhejiang, Jiangsu and Kunming, which can ensure timely and thoughtful after-sales service for customers in the region. Our company's products in various regions involve automobiles, electrical appliances and electric control cabinets. , security doors, file cabinets, steel furniture, radiators, highway guardrails, refined oil drums and other industry customers.

First, the principle of phosphating

The chemical reactions during the phosphating process of iron phosphate membranes are as follows:

4Fe+8NaH2PO4+4H2O+2O2→4Fe(H2PO4)2+8NaOH

In the above formula, Fe is a material to be treated, NaH2PO4 and H2O are main components of the phosphating solution, and 2O2 is an oxidizing agent.

2Fe(PO4)2+2NaOH+2O2→2Fe(OH)3+2NaH2PO4+3H2O

2Fe(PO4)2+6NaOH+2O2→2Fe(OH)3+2NaH2PO4+2NaHPO4+H2O

2Fe(OH)3→Fe2O3+2H2O

Comprehensively available:

4Fe+8NaH2PO4+4H2O+2O2→2FePO4+Fe2O3+2NaH2PO4+3H2O

In the above formula, 2FePO4+Fe2O3 is a phosphate film component.

Second, phosphating classification

1. Classification according to phosphating temperature

High temperature type: 80-90 degrees

Medium temperature type: 50-75 degrees

Low temperature type: 30-50 degrees

Normal temperature type: 10-40 degrees

2, according to the classification of phosphating solution

(1) Zinc phosphating

(2) Zinc-calcium phosphating

(3) Iron phosphating

(4) Manganese phosphating

(5) Composite phosphating: the phosphating solution consists of elements such as zinc, iron, calcium, nickel and manganese.

3. Classification by phosphating treatment method

(1) Chemical phosphating

(2) Electrochemical phosphating

4. Classification by phosphating film quality

(1) Heavy grade (thick film phosphating) The film weight is 7.5 g/m2 or more.

(2) The secondary weight (middle film phosphating) film weighs 4.6-7.5 g/m2.

(3) The lightweight (thin film phosphating) film weighs 1.1-4.5 g/m2.

(4) Sub-lightweight (special film phosphating) film weight 0.2-1.0 g/m2

5. Classified by construction method

(1) Impregnated phosphating

(2) Spray phosphating

(3) Brush phosphating

Third, the role of phosphating

1 Enhance the bonding force between the coating film layer (such as paint coating) and the workpiece.

2 Improve the corrosion resistance of the surface coating of the workpiece after painting.

3 improve the decorative.

Fourth, factors affecting phosphating

1, concentration (total acidity, free acid or acid consumption)

2, temperature: temperature is the key factor of film formation, strict control of temperature, the smaller the temperature change, the better;

3, time: the time is too short, can not produce a phosphate film, the time is too long, the film is coarse and loose;

4. The concentration of iron ions in the solution;

5. The influence of phosphating sediment content;

6. The impact of phosphating equipment;

7. Influence of the surface state of the treated steel.

Fourth, the quality test of phosphate film

1. Appearance inspection of phosphate film

The color of the workpiece after phosphating should be light gray to grayish black or color; the film layer should be crystallized dense, continuous and uniform. The phosphating workpiece has the following conditions or one of them, which are allowed defects: slight water marks, passivation marks, chalking and ash hanging; color and crystallization due to local heat treatment, welding and surface processing conditions Uneven

There is no phosphate film at the weld. When the phosphatized workpiece has one of the following conditions, it is not allowed to be defective: loose phosphating film layer; rust or green spot; local non-phosphorus film (except weld); the surface is heavily ashed.

2, corrosion resistance check

The phosphatized sample was immersed in a 3% sodium chloride solution, and after 1 hour, it was taken out, and no rust stain on the surface was qualified. The longer the rust staining time, the better the corrosion resistance of the phosphating film.

3, coating film corrosion resistance

According to the above test, the coating samples of zinc, manganese and zinc-calcium phosphate films were subjected to a neutral salt spray test for 24 hours, and the coated samples of the iron-based phosphate film were resistant to 8 hours. After the salt spray test, the coating film should be free from blistering, falling off and rusting except for the scratched portion. (Note: The scratched part refers to the width of 0.5mm in the vicinity of any side of the scratch).

4, coating film water resistance

The coating film was immersed in pure water for 48 hours, and air-dried for 2 hours to test its adhesion by the cross-hatch method, which should be ≤2.

V. Composition and nature of oil pollution

For the barrel industry, pre-coating pretreatment includes degreasing and phosphating processes.

1, the composition of oil pollution

(1), mineral oil, vaseline

(2), soaps, animal and vegetable oils, fatty acids, etc.;

(3) Anti-rust additive: the main component of anti-rust oil and anti-rust grease.

2, the nature of oil

(1) Chemical properties

Saponification: animal and vegetable oils (saponification, emulsification, dissolution)

Not saponifiable: mineral oil and petrolatum (emulsified, dissolved)

(2) Physical properties

The greater the viscosity of the oil or the higher the dropping point, the more difficult the cleaning; the polar oil has a tendency to adsorb strongly on the metal of the body, and the cleaning is difficult.

Sixth, metal degreasing method

1. Classification by degreasing ingredients

(1) Degreasing by lye cleaning:

Ingredients: sodium hydroxide, sodium carbonate, sodium silicate, phosphate;

Temperature: 50--80 ° C;

PH value: 12-14.

(2) Emulsifier degreasing

Ingredients: emulsifier TX-10, 6501, sodium alkylbenzene sulfonate, phosphate, etc.;

Temperature: 40--60 °C

PH value: 9-11.

(3) Organic solvent cleaning method

Ingredients: Commonly used paint with gasoline and pine perfume.

(4) Electrochemical degreasing

(5) Ultrasonic cleaning

2, classified by cleaning method

(1) Impregnated

(2) Spray type

Seven, whether the degreasing is thorough inspection method

1, water film method (most commonly used)

The method of determining the degreasing effect by the degreased metal surface, washing with clean water, and observing the water film on the metal surface is called the water film method.

If the water film is evenly continuous, the degreasing effect is good;

If the water film is not completely continuous, the degreasing effect is not very good;

If the water film is not formed at all, the degreasing effect is very poor.

2, wiping method

3. Copper sulfate method

The metal sample was immersed in an aqueous solution of copper sulfate, and after 1 minute, the workpiece was taken out from the solution and rinsed with water. Observe that the copper film is completely uniform, shiny and good in bonding, indicating that the surface of the workpiece is free of oil.

4, oil test paper method

5, determine the rate of degreasing

Due to the complexity of the actual situation, the use of this method has certain limitations.

Eight, common problems and solutions in skimming

1. Phenomenon: The degreasing effect is not good.

(1) Reason: improper selection of degreasing agent; solution: replacement of degreasing agent.

(2) Reason: improper selection of degreasing process; solution: use secondary degreasing, secondary cleaning.

(3) Reason: The surface oil is too thick and the degreasing is uneven; solution: first pre-scrubbing.

(4) Reason: The degreasing temperature is low; solution: increase the degreasing temperature.

(5) Reason: The concentration of degreasing agent is low; solution: increase the concentration to the process range.

(6) Reason: The mechanical effect is not enough; Solution: Increase the injection pressure.

(7) Reason: The nozzle is clogged and the flow is insufficient. Solution: Clean the nozzle regularly.

(8) Reason: The oil content in the working fluid is too high; solution: Periodically change the bath and use secondary degreasing.

(9) Reason: Washing is not complete after degreasing; solution: strengthen water washing, continuous overflow of washing water.

2. Phenomenon: The working fluid foam is high.

(1) Reason: The temperature is too low; Solution: Increase the temperature to the specified range.

(2) Reason: The circulating pump seal is worn and enters the air; Solution: Replace the pump sealing material.

(3), reason: improper selection of degreasing agent; solution: replace the degreasing agent.

(4) Reason: The injection pressure is too high; Solution: Reduce the injection pressure.

(5) Reason: no defoaming process is adopted; solution: defoaming agents such as alcohols and polyethers are used.

3. Phenomenon: There is too much foam in the washing tank.

(1) Reason: The overflow rate of the washing tank is too small; solution: increase the overflow flow.

(2) Reason: The circulating pump seal is worn and enters the air; Solution: Replace the pump sealing material.

(3) Reason: The bath is used for too long; solution: periodically change the bath.

4. Phenomenon: The alkalinity of the washing tank is too high.

(1) Reason: The alkali tank is washed into the tank solution; solution: reform the equipment; avoid sputum.

(2) Reason: The workpiece has too much lye into the water washing tank; solution: change the hanging form and prolong the dripping time.

(3) Reason: The overflow rate of the washing tank is too small; solution: increase the overflow.

(4) Reason: The bath is used for too long; solution: periodically change the bath.

5. Phenomenon: The parts are rusted after washing.

(1) Reason: The process interval is too long; solution: increase the spray between processes.

(2) Reason: The washing time is too long; Solution: It is not allowed to stay here for a long time.

9. Common problems and solutions in phosphating

1. Phenomenon: non-phosphorus or phosphating film is not easy to form

(1) Reason: The surface of the workpiece has a hardened layer; solution: pickling and sandblasting to remove the hardened layer.

(2) Reason: The total acidity is not enough; solution: add phosphating agent.

(3) Reason: The treatment temperature is low; solution: increase the temperature of the phosphating solution.

(4) Reason: Degreasing is not clean or the phosphating time is short; solution: strengthen degreasing or prolong phosphating time.

(5) Reason: The proportion of phosphating solution is out of balance; solution: adjust or replace the phosphating solution.

(6) Reason: The workpiece overlaps or the workpiece faces the surface; Solution: Increase the workpiece gap and avoid the workpiece surface contact.

2. Phenomenon: The phosphating film is too thin

(1) Reason: The phosphating time is not enough; solution: prolong the phosphating time.

(2) Reason: The surface of the workpiece has a hardened layer; solution: pickling and sandblasting to remove the hardened layer.

(3) Reason: The treatment temperature is low; solution: increase the temperature of the phosphating solution.

(4) Reason: The mechanical effect is not enough; Solution: Increase the injection pressure.

(5) Reason: The proportion of phosphating solution is out of balance; solution: adjust or replace the phosphating solution.

3, phenomenon: phosphating surface rust

(1) Reason: The phosphating film grains are too thick or too fine, and the corrosion resistance is reduced; solution: adjust the working fluid index to the process range.

(2) Reason: There is residual acid on the surface of the workpiece; solution: strengthen the water wash.

(3) Reason: The surface of the workpiece is too corrosive; solution: control the pickling process.

(4) Reason: Excessive residue in the phosphating solution has blocked the nozzle; solution: reverse the groove and clean the nozzle.

(5) Reason: The temperature of the phosphating solution is low; solution: increase the temperature of the phosphating solution.

(6) Reason: The spray pressure is large and the nozzle direction is improper; Solution: Check one by one to adjust to normal operation.

Ten, degreasing bath test

1, degreasing bath definition

The total alkalinity refers to the total amount of substances in the water that can neutralize the acid, that is, the total amount of substances that can accept proton H+.

Free alkalinity refers to the total amount of OH- ions contained in water.

2, test

(1) Free alkalinity: Take 10ml of degreasing solution in a conical flask with a pipette, add 4-5 drops of phenolphthalein indicator, the solution is pink, and titrate with 0.1mol/L hydrochloric acid standard solution until the solution becomes colorless. When the time is the end point, the volume (ml) of the titrant consumed is the number of free alkalinity points.

(2) Total alkalinity: Pipette 10ml of degreased liquid degreasing solution into a conical flask, add 50ml of distilled water, add 2-3 drops of bromophenol blue indicator, titrate to yellow with 0.1mol/L hydrochloric acid standard solution That is, the end point, the number of milliliters of hydrochloric acid standard liquid consumed is the total alkalinity point.

Eleven, the phosphating bath test

1. Definition

Total acidity: The sum of free acid and acid in the phosphating solution is called total acidity.

Free acidity: The concentration of free H+ in the phosphating solution is called free acidity.

Acid consumption: The sum of acid consumed by alkaline ions contained in the phosphating solution is called acid consumption.

2, test

(1) Total acidity:

Pipette 10 ml of the treatment solution into a conical flask, add 3-5 drops of phenolphthalein indicator, the solution is colorless, titrate with 0.1N sodium hydroxide standard solution until the solution turns pink and keeps the color for 30 seconds. The number of milliliters of sodium hydroxide consumed at this time is the number of points of total acidity.

(2) Free acidity:

Pipette 10 ml of the treatment solution into an Erlenmeyer flask, add 3-5 drops of bromophenol blue indicator, and titrate the solution with 0.1 mol/l sodium hydroxide standard solution to turn blue and purple to the end point. The number of milliliters of sodium standard solution is the number of points of free acidity of the working fluid.

(3) Acid consumption:

Take 10ml of the treatment solution in the Erlenmeyer flask, add 2-3 drops of bromocresol green indicator, titrate with 0.1N hydrochloric acid standard solution until the solution turns from blue to green, and the amount of hydrochloric acid standard solution consumed is The number of points consumed for acid.

Twelve, the replacement of the bath

1, degreasing tank

After the treatment liquid is used for a long time, the bath liquid will be dirty, and finally the degreasing ability is weakened into an aging state. In this state, the tank liquid supplementing degreasing agent has not achieved the desired effect, so it should be reconstituted. When the ratio of total alkalinity/free alkalinity is greater than 1.8, the bath should be replaced in the entire tank.

2. Phosphating tank

(1) Control Fe2+ content: When the Fe2+ content is >3.5g/L, the bath should be replaced.

(2) Control phosphating slag content: When the phosphating slag content is >5ml/1000ml, it should be replaced.

Note: Phosphating bath is generally not subjected to titration analysis to control the replacement of the bath.

Thirteen, steel barrel pretreatment recommended process

Process: hot water washing → pre-degreasing → main degreasing → washing 1 → washing 2 → phosphating or silane → washing 3 → washing 4 → pure washing → drying → spraying

The process has 9 tanks, which are 2 tanks more than the conventional pretreatment line: hot water washing and pure water washing, but the actual cost can be saved:

1. The first tank is washed with hot water. The following benefits are obtained: at medium and high temperatures (60--80 °C), most of the oil and dust on the surface of the steel drum are removed, and the tank can be replaced in 1-2 days without wasting fat. The agent can save the amount of degreaser because most of the oil has been removed by high temperature in the hot water washing tank;

2. The last tank is washed with pure water. There is no difference from the surface after painting and spraying, but if it is used for salt spray corrosion test, the gap is still large. In addition, there is a big gap in the cleanliness after surface spraying. In the home appliance industry, such as refrigerators, washing machines, air conditioners, etc., it is required that the small particles cannot exceed 3 particles on one side, and the requirements for surface dust are high, so Most home appliance manufacturers use pure water washing in the last process of pre-treatment. The conductivity of pure water is ≤25us/cm, and the conductivity of general tap water is 300--400us/cm. From the development trend of steel drums in China, it is likely to exceed the output of the United States. As the first major producing country, the number of exports will increase sharply, and the quality requirements for products will also increase.

XIV. Prediction of the development direction of pretreatment agents

1. Development direction

No pollution: no phosphorus, no foam, neutral

No heavy metal: It can pass the SGS and ROUS test report (detection does not contain hexavalent chromium, mercury, cadmium, lead ions), and meets the requirements of exporting to Europe, America and Japan.

No emissions.

2, degreaser

Development of phosphorus-free, neutral, non-foaming, heavy metal-free, liquid degreasing agents;

For phosphorus-free, it is actually relatively simple. Currently, there is a non-phosphorus detergent on the market for nearly ten years. But only phosphorus-free is only an indicator to solve the national sewage discharge standards. It still needs environmental protection to deal with other indicators to meet environmental protection requirements. In fact, it needs to be a neutral degreasing agent. The PH value is between 6.0 and 8.0, no foam. There is no heavy metal, so the water washing after the degreasing agent can be directly discharged. If the whole tank is replaced with the degreasing bath, according to the national standard, its COD, BOD and suspended solids still exceed the standard, so it needs to be settled, diluted and filtered before it can be discharged.

The advantage of the liquid degreasing agent is that it is convenient to add and it is not easy to block the nozzle.

3, the type of coating agent

(1) Phosphating: iron phosphating, zinc phosphating

(2) Water-free passivation agent: The coil is passivated with trivalent chromium, not washed after passivation, and a passivation film is formed during the drying process.

(3) Silane passivating agent: It is currently used in the household appliance industry in foreign countries, and it is in the trial production stage in China.

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