galvanized sheet chromate passivation defects
galvanized sheet common pitfalls
1, the substrate defects: hot and cold rolled steel strip in the process, the strip surface will produce some defects in subsequent affect surface quality galvanized zinc coating. Common defects of the substrate has the following types.
a) interlayer (Shell) - in the rolling direction (WR indicates the rolling direction) profile, zinc steel sheet covered with skin-like surface, the alloy layer is not formed, the zinc layer below often have a non-metallic inclusions present in steel or iron oxide substrates;
b) stomata (Blowholes) - the appearance of a bump straight strip coating is an internal strip discontinuity of performance;
c) holes (Holes) - discontinuous holes, is the result of rupture of the material, the thinner the strip, this situation is more obvious. Mainly because of the steel pores, coarse inclusions or the front surface of the strip rolling there is a serious mechanical damage caused;
d) scale defects (Scale defects) - appearance of coating defects such small sunken pits or small bumps. There are two reasons: the oxide scale during hot rolling pits and scars left behind, failed to eliminate when cold-rolled, cold-rolled insufficient pickling or when pressed into the skin caused by oxidation, shape and density of the two are not the same;
e) the original plate abrasions (Starting material scratches) - board scratches appearance of two forms, (1) hot-rolled, pickled, cold rolled scratches, this deficit ditch along the rolling direction of stripes, draw mark at the zinc coating is often due to rapid growth and lead alloy layer where thicker coatings; (2) heat abrasions, showed in the strip running direction short, densely striped uplift;
f) rust and emulsion residue (Rust and coolant stain) - rust material will cause the zinc coating on the surface of the rough-like surface pitting. Emulsion residue on the cold-rolled strip is hot emulsion surface residue decomposition, irregular-shaped dark spots. The residue at such defects on the cold-rolled strip of red rust or emulsion. Emulsion residues are not due to steel cold rolling stage or not wipe dry due. ;
g) cold rolling indentation (Cold rolling marks) - periodically occurring projections or depressions;
h) edge waves, the wave (Edge / central waves) - plate edge wave and the wave is because the cross-sectional view of the substrate caused by defects, but also because of hot-dip galvanizing line roller crown and improper location. Lang also be by the side edge portion is too thick, trimming blade pitch adjustment inappropriate or steel edge and the fixing member, such as guides, the contact between the cause;
i) with volume uplift (Ridges) - also known as ribs, coil edge uplift, is a former board transversely a reflection of uneven thickness, hot defect, cold can not be completely eliminated;
j) tensile strain mark (stretcher strain marks) - due to the aging of the steel sheet or strip surface lead slip line, "orange peel" and detrimental to the appearance of defects.
2, the hot-dip galvanizing process caused by defects
a) furnace roller printing (Furnace roll pimples) - This is a coating defect pit or flake indentation, Genesis is the furnace roller surface sediment adhesion, sometimes these things are taken away strip adhesion, resulting in steel with surface projections or depressions;
b) iron oxide scales (Flakes of scale) - the appearance of plaque, a thick coating of these plaques, in the future the process will occur crack and fall off. This flaw is due to an annealing furnace at scale remain in the strip formed on the surface together with the steel into the zinc pot, so the scale adhesion plaques also stuck liquid zinc. There may be zinc oxide snout liquid surface is formed embedded within the zinc layer.
c) scum (Dross) - along with the distribution of the residue in the galvanizing layer formed longitudinally uneven inclusions, its causes are small granules at the snout of dross or zinc ash out of the strip;
d) coating stripes (Ribs or coaling sag) - the performance of the strip along the width direction of the coating was striped irregular thickening of zinc solution Cause usually aluminum content is too high (greater than 0.25%), or strip the zinc pot temperature is low, such defects during the alloying treatment can be eliminated.
3, after the process of defect formation
a) chromate passivation defects - zinc chromate passivation layer of the surface of brown marks. They are likely to occur over the entire strip width and length of the strip shape, elongated or irregular spots. This flaw is due to surface wear wringer rolls or squeeze pressure is not enough, not sufficient due to chromate solution squeezed after chromate.
b) White rust (White rust) - galvanized layer surface unevenness of an off-white powder, it is possible to cover the entire surface of the steel sheet in an extreme case. White rust is due to the surface layer of zinc corrosion products caused by moisture.