The surface galvanized layer should be complete, uniform and of standard thickness. The evaluation of the appearance quality of the galvanized layer is very important. The following lists the commonly used galvanized layer surface conditions for galvanized steel pipes.
Gray and dark gray spider web
The chemical elements of the steel pipe, such as C, Si, P and Mn, will cause the Fe-Zn alloy to react as the content increases, producing an iron-zinc alloy layer on the coating surface. Especially if it increases, the reaction will be accelerated by the mutual iron-zinc melting, forming Gray or even black mesh on the surface. The black coating layer is thicker than the bright coating, and its anti-corrosion ability is not as good as the bright coating, but it may not be as good in terms of perception. Hot-dip galvanized pipes inevitably turn to dark colors in a short period of time.
ASTM123, JISH8641, BS729 and other standards stipulate that the gray coating does not affect the corrosion resistance of the galvanized layer. The new version of the BS EN ISO 1461:1999 standard is more clear and adds: "As long as the minimum coating thickness is greater than the pipe surface galvanized, even in the presence of dark gray areas (such as dark gray grid pattern areas) or some rough surfaces should not considered grounds for rejection"
2 Coating surface roughness
●Rough zinc layer is usually caused by uneven growth of zinc-iron alloy. Due to the uneven distribution of chemical elements in the steel pipe and the different thicknesses of the zinc-iron alloy layer produced, the surface after galvanizing is rough. However, it does not affect the use. However, if the customer has special requirements in advance, he should consult a galvanizing expert for advice and choose materials with uniform composition and good surface gloss.
●Flow traces of zinc and water. During the first minutes of immersion in a galvanizing bath, a zinc-iron alloy layer rapidly grows on the surface of most commonly galvanized steel. Excessive zinc solution will accumulate in areas where the zinc is not smooth, forming traces of zinc and water. This will not affect the use of the coating, but will form a thicker coating to enhance anti-corrosion capabilities. If the customer has special requirements in this regard, the galvanizing manufacturer should be consulted for special design and treatment of zinc.
●Zinc tumors. Mainly derived from Zine Zine (a suspended Zn-Fe alloy galvanized steel pipe adheres to the surface), when the Zine tumor is removed, the galvanized layer can be fully revealed. Its corrosion resistance is equivalent to that of ordinary zinc layers. A small number of scattered zinc nodules are not harmful to the coating surface, but excessive and excessive zinc tumors can 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 overlapping or damp, poorly ventilated storage and transportation. The product has minimal corrosion on the galvanized layer and does not affect the service life of the galvanized layer. If the thickness of the galvanized layer is still in accordance with the standard requirements after removing the white rust, it will not affect the use of the steel pipe, which is acceptable.
The method to eradicate white rust is: first, a dry and ventilated environment must be provided during storage and transportation, avoid overlapping stacking, and cover the upper part of the steel pipe; secondly, provide the steel pipe with a passivation solution after galvanizing.
