Surface treatment process design before steel drum coating

At present, domestic materials used for manufacturing steel drums are mainly cold-rolled steel sheets. For the coating of steel drums, due to the existence of certain oil stains on the steel surface, in order to guarantee the coating quality of steel drums, especially for drums and inner steel drums for powder coating, steel drums must be pre-treated. . Through pre-treatment, the adhesion of the steel drum and the coating film can be effectively improved, thereby protecting the steel drum from corrosion damage.

At present, many domestic companies have steel drum coating pre-processing capacity, but the treatment effect and processing cost are very different, among which there are still many high-cost and poor-effective treatment processes. The author draws lessons from foreign advanced steel barrel processing technology design, and combines the production characteristics of domestic large steel barrel enterprises to optimize the design of steel barrel coating pretreatment process for reference.

First, the steel drum coating pre-treatment process design

The typical pretreatment process for steel drum coating is:

The steel drum coating pretreatment equipment is designed as a seven-station combined cleaning production line (including chamber body, liquid reservoir, spray system, heating system, and filtration system). Since the pre-treatment of steel drums is performed in an automatic line, the pre-treatment form adopts a through spray type, and the process speed of the pre-treatment conveyor chain is 1.25 m/min.

Second, steel drum coating pretreatment process design

1, degreasing treatment process

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

(1) Bath (4000L):

Add fresh water to the tank 3/4, warming to about 40 °C; slowly add 100kg C1-364G degreasing solution, slowly stirring; then add water to 4000L, heated to 55 ~ 60 °C.

(2) Processing conditions:

Alkalinity 27 PT

Temperature 55~60°C

Time 2min

(3) Inspection methods:

Take 10ml of treatment solution, add 5 drops of phenolphthalein indicator, with 0.05mol H2SO4 titration droplets until the pink disappears, the consumption of titrants in milliliters is the alkalinity point.

(4) Replenishment and Adjustment:

If the treatment solution is tested and found to be below the alkalinity required for the treatment conditions, a C1-364G degreasing solution must be added, and the alkalinity of the bath can be increased by 1PT per 8 kg added. If the agent is added, degreasing effect can be obtained. Still not easy to change, the bath has aged and needs to be replaced.

2, table treatment process

The surface conditioning agent used before the treatment of the phosphate coating on the steel drum surface is PL-Z. The surface adjustment of the steel drum is: First, the surface activity of the steel drum is uniformized; second, the phosphating time is shortened by more than 1 time; Third, the crystallization of the phosphating film is fine and uniform; and fourth, the phosphating film is enhanced in adhesion and corrosion resistance. The table slot adjustment volume is designed to be 1.8m3 (1800L).

(1) Bathing:

Add fresh water to the table tank approximately 80%; start the spray pump or circulating pump, gradually add the table adjustment agent, to the required amount of 5.4kg, and make it dissolve; add fresh water to the treatment volume of 1800L.

(2) Processing conditions:

Temperature at room temperature

1min

Treatment solution pH 7.5~8.5

The PL-Z conditioning agent is a white powder with a white turbid solution. The presence of serum will reduce the effect. In order to maintain the state of white corrosion, the treatment liquid needs to be agitated.

(3) Supply and adjustment:

When the cumulative treatment area reaches 1000m2, additional 0.9kg PL-Z table adjustment liquid must be added.

If the pH drops, add appropriate amount of sodium hydroxide to adjust. Sodium hydroxide is pre-dissolved in a suitable amount of water and added in solution. When sodium hydroxide is pre-dissolved, it will heat up severely and should be gradually added to water.

After a certain period of time, the treatment solution will age, and the effectiveness will decline. When the PL-Z is added, there is no significant effect, and the treatment solution must be renewed.

3, phosphating process

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

(1) Use of pharmaceuticals:

PB-L47 bath builder, PB-L47 supply agent;

Anhydrous sodium carbonate (soda), accelerator 131.

(2) Testing drugs and equipment:

Phenolphthalein, bromophenol blue indicator;

0.1mol NaOH titrant;

0.042N potassium permanganate, 50% sulfuric acid reagent;

50ml brown and white pipettes, test strips.

(3) Bath (4000L):

Add clear water to the phosphating bath and add 280kg of PB-L47 builder bath while stirring; keep stirring, add 4.0kg of anhydrous sodium carbonate pre-dissolved in 20L of water, add fresh water to the liquid 4000L and stir; The acidity of the liquid; before starting the treatment, 3 kg of accelerant 131 is added and 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) Supply and Adjustment:

Total acidity: Phosphate treatment is continued, and the concentration of the treatment solution will decrease, mainly as a decrease in total acidity, which needs to be regularly tested and the prescribed total acidity is constantly maintained. PB-L47 replenisher can be used to adjust the total acidity. For every 6.4 kg of supplement added, the total acidity rose by about 1 point.

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

Free acidity: If free acidity changes, soda can be used to adjust. In the 4000L phosphating bath, the free acidity decreased by 0.1 point for every 212g of soda added.

(6) Detection method:

Total Acidity: Use a pipette to win the 10ml treatment solution into a beaker, add 5 drops of phenolphthalein indicator, titrate with 0.1mol NaOH to the sample pink, at this time the consumption of titrant ml is The total acid number of points.

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

Accelerator concentration: First dip the test paper and observe whether the test paper is discolored. When the test paper becomes red, the accelerator 131 is added to the treatment liquid, and when the treatment liquid cannot change the color of the test paper, the accelerator concentration is detected.

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

Third, the conclusion

In short. In order to ensure the coating quality of steel barrels, it is necessary to timely control the concentration and technological conditions of the pretreatment fluid, master the correct operation methods, and constantly improve and improve the knowledge and theoretical level of the pretreatment technology in practice to solve the practical problems in the production process. Ensure product quality and grade.