Yesterday when I went to the warehouse to stock goods, I ran into a guy who made guardrails. He pointed to a batch of hot-dip galvanized square pipes and frowned, saying, "Brother, why are all the zinc nodules on the surface of these pipes? They look like they've grown lumps. Can they be stopped by spray painting
This is a typical example of being "educated" by low-priced goods. In the steel industry, hot-dip galvanized square tubes are considered one of the varieties that appear to have a low threshold but actually have the deepest water filling. Today, let's break away from textbook style parameters and use the perspective of frontline practitioners to talk about how to distinguish the quality of hot-dip galvanized square tubes and under what circumstances you should not buy them.
1、 Don't be fooled by the three words' hot-dip galvanizing ', the difference in craftsmanship is almost negligible
Many people think that as long as it is hot-dip galvanized, the rust prevention period is the same. absolutely wrong.
1. Blowing plating vs hanging plating (a secret phrase in the old martial arts world)
Blow coating (air knife galvanizing): 90% of the so-called "hot-dip galvanized square tubes" on the market now use this process. After the steel pipe passes through the zinc liquid, the excess zinc is blown away with high-pressure gas. The advantages are fast speed, low cost, and relatively flat surface; The disadvantage is that it has poor coverage ability for the inner corners of complex cross-sections such as pipes, and the zinc layer is thin, usually only 60-80 microns according to national standards.
Hanging plating: Hanging pipes one by one on hooks and dipping them in zinc. The advantages are a thick zinc layer (up to 100-150 microns), the ability to attach zinc to the inner corners, and excellent adhesion; The disadvantage is low efficiency, high cost, and the price is usually 800-1200 yuan/ton more expensive than blown plating.
My suggestion: If exposed outdoors all year round, especially near the seaside or chemical plants, try to hang plated pipes as much as possible; Indoor dry environment, sufficient blown coating.
2、 Visual Poison Detection: Three Tricks to See Through Poor Quality Hot Dip Galvanized Square Tube
Without using a spectrometer or a thickness gauge, the master can screen out half of the defective products with his eyes and nails.
First move: Look for "flow marks" and "zinc tumors"
Pick up the pipe and spin it around. If there are obvious traces of tears flowing down on the surface, or if there are large bumps (zinc nodules) in certain areas, it indicates that the temperature of the zinc solution is too high or the speed is not well controlled. This kind of pipe is not only ugly, but the spray on the back of the zinc tumor will definitely cause it to peel.
Second move: Cut the interior angle
This is the dead end of hot-dip galvanized square tube. Turn the tube towards the light and look into the inner corner. A good hanging plated tube, the inner corners are also shiny silver; Poor blown plated pipes have dusty interior corners, and even a circle of uncoated black seams can be seen. Attention friends who make guardrails and fences, the inner corners have rusted, and the entire shelf life will be cut in half.
Third move: Bend and listen to the sound
Find a scrap head and fold it hard. The high-quality hot-dip galvanized layer has excellent toughness and will deform along with the steel pipe without cracking; The inferior galvanized layer is very brittle, and it will "break" with a loud sound and fall off directly, exposing the black iron inside.
3、 Three questions that must be asked to suppliers during procurement
Don't be embarrassed, you are the first party, and asking for more details is for your own benefit.
Is it ASTM A500 or GB/T 3091? ”
The American standard has higher requirements for galvanized layers (average 85 microns) than the national standard (average 65 microns), but of course, the price is also expensive. Don't blindly buy money for the national standard and expect to achieve the same effect as the American standard.
Which company uses zinc ingots
Regular large factories use 0 # zinc ingots (purity 99.995%). Some small workshops use 1 # zinc or even recycle zinc, which contains a lot of impurities. The plated layer becomes dark and black, and white rust appears after six months.
Can we pass the salt spray test
Those who dare to boast that they can pass the 72 hour neutral salt spray test (without red rust) are basically reliable large factories. If the other party is stuttering, then don't talk about it.
4、 The 'archenemy' of hot-dip galvanized square tube: Do not use it in these situations
Although hot-dip galvanized square tube is good, it is not a panacea.
Underground project: The acidity, alkalinity, and salinity in the soil are too complex, and the hot-dip galvanized layer will quickly be corroded to the point of exhaustion. This situation requires 3PE anti-corrosion or cement mortar lining.
Internal of precision instruments: The hot-dip galvanized surface is relatively rough with zinc nodules, which is not suitable for equipment with extremely high appearance or strict size requirements.
High frequency welding scenario: The hot-dip galvanized layer has a low melting point, which can easily generate a large amount of zinc vapor during high-frequency resistance welding, leading to porosity in the weld seam. If welding is necessary, be sure to use pipes with the "pre zinc scraping" process, or polish off the zinc layer at the weld before welding.
5、 A little truth from the old steel trade
Hot dip galvanized square tube is the "labor protection shoe" in the industrial industry, with sturdy leather, durability, and high cost-effectiveness. But the market is mixed with good and bad, from "black pipes" priced at a few thousand yuan per ton to "precision plated pipes" priced at over ten thousand yuan, all of which are called hot-dip galvanizing.
Remember one sentence: You get what you pay for, which is fully reflected in galvanized pipes. Don't let the entire construction project rust for two or three years just to save a few hundred yuan per ton. The cost of rework will be enough for you to buy several batches of good pipes.
