The total zinc coating condition applied depends mainly on the mass and thickness of the galvanized steel tubing .Usually the surface of the galvanized layer should be complete and uniform and with a standard thickness. The evaluation of the appearance quality of the galvanized layer is very important. The following listed the common surface conditions of the galvanizing layer for galvanized steel pipe.
1 Gray and dark-gray cobwebbing
The chemical elements of the steel tube such as C, Si, P and Mn will cause Fe-Zn alloy reaction with the increase of the content, produced zinc-iron alloy layer on the coating surface, especially with Si increased, which accelerate the reaction of iron-zinc melt mutually, formed on the surface of gray even the black net. The black coating layer is thicker than the bright coating, the corrosion protection and prevention ability completely inferior to the bright coating, but will be more resistant to corrosion but may not pretty sensually. Hot-dipped galvanized pipe will inevitably turn to dark color after a short period of time.
ASTM123, JISH8641, BS729 and other standards specified that the gray coating does not affect the corrosion resistance of the galvanized layer. The new version of the BS ENISO1461:1999 standard is more clear and added the following: "as long as the minimum coating thickness greater than the pipe surface to be galvanized, even in the presence of dark gray areas (such as dark gray mesh pattern area) or some uneven surface should not be considered a reason for rejection"
2 Coating surface roughness
●The rough zinc layer is usually due to the uneven growth of Zn-Fe alloy. Because the uneven distribution of chemical elements in steel pipe and the different thickness of zinc iron alloy layer generated, make the surface roughness after galvanization. But it does not affect the use, but if the customer has a special requirement in advance, it should ask the advice of the galvanizing expert and choose the material with uniform composition and good gloss on the surface.
●Zinc and water flow traces.During the first minute of immersion in the galvanizing bath,zinc-iron alloy layers grow rapidly on the surface of the steels which are most commonly galvanized.The excess zinc solution will accumulate in some areas where zinc is not smooth, creating zinc and water traces. This will not affect the use of the coating, on the contrary will form a thick coating to enhance the anti-corrosion ability. If the customers have special requirements in this respect, they should consult with the galvanizing manufacturers for the special design and treatment of zinc.
●Zinc tumor. Mainly from the zinc zincilate (a kind of suspended Zn-Fe alloy galvanized steel pipe adhesion on the surface), galvanized layer can be showed completely when the zine tumor removed. Its corrosion resistance is equivalent to that of the general zinc layer. A small amount of dispersed zinc nodules is not harmful to the surface of the coating, but too much and too much zinc tumor will seriously damage the smoothness and appearance of the coating.
3 White rust
White rust is mainly a corrosion product of zinc - Alkaline Zinc Oxide. White rust usually occurs in heavy overlapped or moist, poorly ventilated storage and transportation. The corrosion product to the galvanized layer is minimal, and it does not affect the service life of the galvanized layer. If the thickness of galvanizing layer still in accordance with the standard requirements after the white rust removel, is it does not affect the use of the steel pipe, which is acceptable.
The way to eradicate white rust is: firstly, we must provide dry and ventilated environment during storage and transportation, avoid overlapping stacking, and cover the upper part of the steel tubing; secondly, provide passivation solution after galvanization for steel pipe.