Surface treatment design before steel drum coating

At present, the materials used in the manufacture of steel drums in China are mainly cold-rolled steel sheets. For the coating of steel drums, in order to ensure the quality of the steel drum, in order to ensure the quality of the steel drum, especially for the steel drum and the inner steel drum for powder coating, the steel drum must be pretreated. . Through pretreatment, the adhesion between the steel drum and the coating film can be effectively improved, thereby protecting the steel drum from corrosion damage.

At present, many domestic enterprises have the pre-treatment ability of steel drums, but the treatment effect and treatment cost are very different, and there are still many treatment processes with high cost and poor effect. The author draws on the advanced foreign steel barrel process design and combines the production characteristics of domestic large steel drum enterprises to optimize the design process of steel drum pre-painting for reference.

First, the process design of steel drum pre-treatment process

The typical pretreatment process for steel drum coating is:

The steel drum pre-treatment equipment is designed as a seven-station joint cleaning production line (including chamber body, liquid storage tank, sprinkler system, heating system, filtration system). Since the pre-coating treatment of the steel drum is carried out in the automatic line, the pre-treatment form adopts the through-type full spray form, and the process speed of the pre-treatment conveyor chain is 1.25 m/min.

Second, the design of steel drum pre-treatment process

1. Degreasing treatment process

The degreasing of the steel drum uses C1-364G degreaser. The volume of the degreasing tank in the workshop is designed to be 4m3 (4000L).

(1) Building a bath (4000L):

Add 3~4 of water to the tank and raise the temperature to about 40 °C; slowly add 100kg of C1-364G degreasing solution and stir slowly; then add water to 4000L and heat to 55~60 °C.

(2) Processing conditions:

Alkalinity 27 PT

Temperature 55~60°C

Time 2min

(3) Inspection method:

Take 10ml of treatment solution, add 5 drops of phenolphthalein indicator, titrate the droplets with 0.05mol of H2SO4 until the pink color disappears, and the milliliter number of the consumption titration solution is the alkalinity point.

(4) Replenishment and adjustment:

If the alkalinity is lower than the alkalinity required for the treatment conditions, it is necessary to add C1-364G degreasing solution. For each 8kg, the alkalinity of the bath can be increased by 1PT. If the agent is added, the degreasing effect is added. Still not good, the bath has aged and needs to be replaced.

2, table adjustment process

The surface conditioner used before the phosphate coating on the surface of the steel drum is PL-Z. The surface adjustment of the steel drum is: first, the surface activity of the steel drum is uniform; secondly, the phosphating time is shortened by more than one time; Third, the crystal of the phosphating film is fine and uniform; fourth, the adhesion of the phosphating film and the anticorrosive property are enhanced. The shop table tank capacity is designed to be 1.8m3 (1800L).

(1) Building a bath:

Add about 80% of the water to the metering tank; start the spray pump or circulating pump, gradually add the surface conditioning agent to the required amount of 5.4kg, and dissolve it; add water to the treatment liquid volume of 1800L.

(2) Processing conditions:

Temperature normal temperature

Time 1min

Treatment liquid pH 7.5~8.5

The PL-Z surface conditioning agent is a white powder and the solution is white turbid. If there is clear liquid, it will reduce the effect. To maintain a white etch, the treatment fluid needs to be agitated.

(3) Replenishment and adjustment:

When the cumulative treatment area reaches 1000m2, 0.9kg PL-Z metering must be added;

If the pH value drops, add an appropriate amount of sodium hydroxide to adjust. Sodium hydroxide is pre-dissolved in an appropriate amount of water and added as a solution. Sodium hydroxide will be strongly exothermic when it is pre-dissolved and should be added to the water gradually.

The treatment liquid will age after a certain period of time, and the effectiveness will decrease. When the supplemental PL-Z has no obvious effect, the treatment liquid should be discharged and updated.

3. Phosphating treatment process

The PB-L47 zinc phosphate film forming agent is selected for the phosphating agent before the coating of the steel drum. The volume of the phosphating tank in the workshop is 4m3 (4000L).

(1) Use of medicine:

PB-L47 bathing agent, PB-L47 replenisher;

Anhydrous sodium carbonate (soda), accelerator 131.

(2) Testing drugs and equipment:

Phenolphthalein, bromophenol blue indicator;

0.1 mol of NaOH titration solution;

0.042N potassium permanganate, 50% sulfuric acid reagent;

50ml brown and white pipette, test paper.

(3) Construction bath (4000L):

Add clear water to the phosphating tank and fill it with 280kg of PB-L47 bathing agent while stirring; keep stirring, add 4.0kg of anhydrous sodium carbonate pre-dissolved in 20L water, add clear water to the liquid volume 4000L and stir well; The acidity of the liquid; before starting the treatment, 3 kg of the accelerator 131 is not added, and it is uniformly mixed with the treatment liquid.

(4) Processing conditions:

Total acidity 30±2 points

Free acidity 3.5±0.5 points

Acid ratio (total acidity / free acidity) 8.5~10

Accelerator concentration 2.0 ± 0.5 points

Temperature 50~55°C

Time 2min

(5) Replenishment and adjustment:

Total acidity: Continuous phosphating treatment, the concentration of the treatment liquid will decrease, mainly as the total acidity decreases, and the required total acidity should be regularly checked and maintained. The total acidity can be adjusted with a PB-L47 replenisher. For every 6.4 kg of supplement, the total acidity increased by about 1 point.

Accelerator concentration: Even without phosphating, the accelerator concentration will decrease due to time effects. Concentration detection must be performed and adjusted to the specified concentration before starting the treatment. The accelerator can be added to adjust the concentration. According to the amount used, 1.5 kg of the accelerator is added in the 4000 L phosphating bath, and the concentration is increased by 1 point.

Free acidity: If the free acidity changes, it can be adjusted with soda. In the 4000 L phosphating bath, the free acidity decreased by 0.1 point for each 212 g of soda added.

(6) Detection method:

Total acidity: Use a pipette to win 10ml of treatment solution into a beaker, add 5 drops of phenolphthalein indicator, and titrate with 0.1mol sodium hydroxide until the sample is pink. At this time, the milliliter of the titrant is used. The total acidity points.

Free acidity: Take 10ml of treatment solution into the beaker with pipette, add 3 drops of bromate blue indicator, titrate with 0.1mol sodium hydroxide until the sample turns from yellow to purple blue, then use the titration solution The number of milliliters is the number of points of free acidity.

Accelerator concentration: first dip with test paper to observe whether the test paper is discolored. When the test paper turns red, the accelerator 131 is added to the treatment liquid, and when the treatment liquid cannot discolor the test paper, the accelerator concentration is detected.

Pipette 10 ml of the treatment solution into a beaker, add 10 to 20 drops of 50% sulfuric acid, titrate with 0.042 N potassium permanganate titration, titrate until the sample is potassium permanganate-specific pink 10s does not disappear, The number of milliliters of titrant used at this time is the accelerator concentration point.

Third, the conclusion

In short. In order to ensure the quality of steel drum coating, it is necessary to timely control the concentration and process conditions of the pretreatment liquid, master the correct operation method, constantly improve and improve the knowledge theory level of the pretreatment technology in practice, and solve the practical problems in the production process. Guarantee the quality and grade of the product.