Quality varies widely across the market. If you run into any of the following, pay extra attention.
1. Creases on the surface
Creases are folds running lengthwise along the tube. They usually happen when small mills, chasing output, apply excessive reduction during rolling, causing "ear" defects on the billet that become creases in subsequent passes. A tube with creases is prone to cracking when bent, and its strength drops noticeably.
2. Pitting on the surface
Pitting looks like small, uneven craters across the surface. It's caused by badly worn roll grooves. Some unregulated mills cut costs by using roll grooves past their service life — the dimensions are already off, and the tubes come out pitted.
3. Scabs on the surface
Scabs usually come from two sources: uneven material with lots of impurities in the steel, or the mill's guide and protection equipment being too crude and prone to sticking, so impurities scratch the surface during rolling.
4. Cracks on the surface
Cracks are mostly a billet problem. Low-quality tubes often use porous, crude billets that crack under thermal stress during cooling, and the cracks remain after rolling. This kind of tube should never be used in load-bearing structures.
5. Lots of surface scratches
Production lines with crude equipment tend to produce burrs, which scratch the tube surface as it moves through the line. Shallow scratches affect appearance; deep ones directly reduce strength, especially in areas under tension — inspect those carefully.
6. Dull color, slightly reddish
A normal galvanized tube has a metallic luster. Low-quality tube looks dull, even slightly reddish. The root cause is rolling temperature not meeting standard — small mills estimate temperature by eye and can't guarantee rolling within the specified austenite range, so the steel's properties naturally fall short.
7. Thin, low transverse ribs
Look at the tube's transverse ribs. If they're thin and low, it's usually from incomplete filling. This affects the tube's interlock and load-bearing performance, which is a real hazard when used as a structural component.
8. Oval cross-section
A square tube's cross-section should be, well, square. If it's oval, the mill often cut corners by reducing thickness too aggressively in the first two finishing passes — the result is lower strength and outer dimensions that don't meet standard.
9. Uneven cut ends
Good material uses uniform steel and a flying shear with enough tonnage, so the cut end is smooth and even. Low-quality tube often has chips falling off the cut end, an uneven surface, and little metallic luster. Also, some mills have poor cutting processes, leaving obvious bulges at the head and tail ends — you can spot this just by looking at both ends when receiving goods.
