Countermeasures for Surface Shrinkage Defects in Spraying Process

Countermeasure against surface shrinkage defects in spray process：

(1) high quality cold rolled sheet with thickness less than 1mm

No pinholes will appear after painting. If there is a crack, because there is air in the crack, coating solidification may cause coating interruption and discontinuity, that is, large pores;

A porosity is caused by rust or dirt on the surface of the steel plate.

B the pores at the tip of the needle were caused by pre-treatment residue or phosphating and ash deposition.

(2) cold rolled sheet with thickness greater than 1mm

The breakage or discontinuity of the coating at the bend indicates that the steel plate is damaged (grooves appear on the inner side of the bend), and there is air in the grooves.

A If there is a grain-size pore at the bend, indicating that the degree of damage is not serious or the stain on the steel plate is not cleaned up, at this time may accompany the appearance of surface particles;

B If there are pin-sized pores in the bend, it shows that there are contaminants in the bend, such as oil removal is not clean, pretreatment residue, phosphating hanging ash, etc., at this time will be accompanied by smaller surface particles.

(3) for hot-rolled plates, due to their thicker oxide scales, they need to be removed before phosphating.

Porosity after coating is due to over-corrosion of steel plate pickling or quality problems of steel plate itself; if there are pinholes, it is due to the quality of steel plate itself.

(4) there is no coating discontinuity at solder joints.

A if there is a hole in the size of rice grain, it indicates that there is void welding or sand holes or rust at the solder joints.

B If there are pin-sized pores, indicating that the solder joint oxidation skin thick, not polished or not thoroughly polished, or when phosphating hanging ash here.

C workpiece with copper and steel welded joints, if the flux cleaning is not clean, coating will appear after coating discontinuity or grain size pores.

(5) defects such as blowholes and pinholes after coating of iron castings.

Especially when the iron castings are pickled, this defect is more serious, because it is unavoidable that there are pores and sand holes in the casting, so the pretreatment of iron castings is often achieved by sandblasting or shot blasting.

(6) iron and steel parts after desulfuring by sulfuric acid

Especially after many times of remodeling of the steel workpiece, due to the substrate itself has been corroded to varying degrees, so often appear after painting pores and pinholes, very difficult to solve.

It can be solved by secondary dehydration or by increasing the thickness of the phosphating film, and can be coated with ash if necessary.

2.2 zinc and zinc alloy workpieces

(1) hot-dip galvanizing and galvanized workpiece

Because of the compact galvanized layer, pinholes do not appear after coating, and cold galvanized sheet, especially the poor quality of cold galvanized sheet, zinc coating is relatively loose, sometimes there will be pinholes after coating.

A If the coating breaks or discontinues at the bend, it indicates that the steel plate is damaged (there is a groove in the inner side of the bend), and there is air in the groove.

B If there are rice grain size pores at the bend, indicating that the degree of damage is not serious or the dirt on the substrate is not cleaned, or the galvanized layer is corroded when degreasing, that is, white spots appear after dehydration, at this time may accompany the appearance of surface particles;

C If there are pin-sized pores in the bend, indicating that there are contaminants in the bend, such as oil removal is not clean, pretreatment residue, phosphating hanging ash or galvanizing coating, although corrosion, but no white spots, at this time will be accompanied by smaller surface particles.

(2) there is no coating discontinuity at solder joints.

A if there is a hole in the size of rice grain, it indicates that there is void welding or sand holes or rust at the solder joints.

B If there are pin-sized pores, indicating that the solder joint oxide coating is thick, not polished or not thoroughly polished, or when phosphating hanging ash here, this will be accompanied by smaller surface particles.

(3) for zinc and zinc alloy parts after removing the plastic from sulfuric acid.

Especially after many times of plastic removal of zinc and zinc alloy workpiece, because the galvanized layer has been completely corroded, and the substrate itself has been corroded to varying degrees, so after painting frequently appear pores and pinholes, very difficult to solve.

It can be solved by secondary dehydration or by increasing the thickness of the phosphating film, and can be coated with ash if necessary.

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2.3 aluminum and aluminum alloy workpieces

(1) Pure aluminium workpieces, especially sheets, generally have good surface quality, and no pinholes will appear after coating, but pinpoint size pores often appear, mainly due to poor pretreatment and other processes.

(2) Although the surface of die-casting aluminum alloy workpiece is relatively smooth, it is unavoidable that there are pores and sand holes in the die-casting process, so the defects such as pores and pinholes often appear after coating.

Serious will lead to coating interruption or discontinuity, especially in the pretreatment of alkali corrosion and light out process more serious.

(3) Porosity and pinholes are often found in aluminium alloy castings due to the unavoidable occurrence of pores and sand holes during casting.

Serious will lead to coating interruption or discontinuity, especially in the pretreatment of alkali corrosion and light out process more serious. Casting or die-casting of aluminum and aluminum alloy parts is often done by sand blasting or shot blasting.

(4) For aluminum and aluminium alloy parts which have been desulphurized by sulfuric acid, especially after several times of desulphurization, because the substrate itself has been corroded to varying degrees, it is difficult to solve the problem that pores and pinholes frequently appear after coating.

Two dehydration methods can be used, and when necessary, smear with atomic ash can be used to solve them.

If organic solvent is used to remove plastic, coating shrinkage and other surface problems may occur after coating, so aluminum and aluminum alloy should be carefully used organic solvent.