The steel pipe billet is too corroded, and a thick iron oxide layer appears on the surface. Because the elements in the steel matrix are not necessarily uniform, some places are deeply corroded, and some places are shallowly corroded. After pickling, large uneven pits appear. Such a surface still leaves pits after hot-dip galvanizing.

When the steel pipe is pickled, many pits are caused by over-pickling, especially in the welds. The action of micro-batteries causes this. Too large pits cannot be filled during hot-dip galvanizing.

The carbon (C) content, iron carbide (Fe3C), silicon, aluminum, copper, phosphorus, sulfur, and nitrogen in the steel pipe matrix will cause different dissolution rates during pickling and cause pits. This situation is very easy to occur in basic converter steel, so it is also easy to cause over-pickling.

The pickling steam scars formed during pickling are ring-shaped or bird's nest-shaped, which are easy to cause pits.

1. Nominal diameter is 25mm, and the corresponding outer diameter is about 33.5mm.
2. Nominal diameter is 32mm, and the corresponding outer diameter is about 42.4mm.
3. Nominal diameter is 40mm, and the corresponding outer diameter is about 50.8mm.
4. Nominal diameter is 50mm, and the corresponding outer diameter is about 61.8mm.
5. Nominal diameter is 65mm, and the corresponding outer diameter is about 76.2mm.
6. Nominal diameter is 80mm, and the corresponding outer diameter is about 88.9mm.
7. Nominal diameter is 100mm, and the corresponding outer diameter is about 114.3mm.
8. Nominal diameter is 125mm, and the corresponding outer diameter is about 139.7mm.
9. Nominal diameter is 150mm, and the corresponding outer diameter is about 168.3mm.
10. Nominal diameter is 200mm, and the corresponding outer diameter is about 219.1mm.

The nominal diameter of hot-rolled seamless steel pipe can be up to 600mm, while the maximum nominal diameter of cold-rolled seamless steel pipe is 200mm.

Appearance:
The appearance of hot-dip galvanized square tubes is slightly rougher than that of cold-dip galvanized square tubes; the appearance of cold-dip galvanized square tubes is relatively smooth and bright.

Zinc layer thickness:
There is a big difference between cold-dip galvanized square tubes and hot-dip galvanized square tubes in the galvanizing process; the corners of cold-dip galvanized square tubes are prone to "electrical burning". The color of this part is dark and the zinc layer is thicker. It is easy to form a current dead angle in the inner corner to produce an undercurrent area. The zinc layer in this area is thinner, and there is no zinc layer attached to the inner wall of the cold-dip galvanized square tube.
The cold-dip galvanized square tube has no zinc nodules, agglomerations, etc. as a whole, but the overall zinc layer is thinner, and the thickness of the outer wall zinc layer is 5μm-10μm.
The inner and outer walls of the hot-dip galvanized square tube are both attached with zinc layers, there will be water marks and a few zinc nodules, but the overall zinc layer is thicker, and the thickness of the zinc layer on the inner and outer walls of the square tube is more than 70μm.

Color:
Cold-dip galvanized square tubes are yellowish-green, bluish-white, or slightly colorful at a certain angle to the sun. Hot-dip galvanized square tubes are silvery white at a certain angle to the sun.