Warning in the application of galvanized steel sheets - risks in livestock industry

With the upgrading of manufacturing and the increase in the output of private galvanizing plants, the price of galvalume products has become cheaper and cheaper. Many applications that originally used pure zinc steel sheets, such as factory buildings, ventilation ducts, color-coated substrates, etc., have begun to switch to use them. Galvalume steel plate. In Southeast Asia, Eastern Europe, Central and South America and other places, my country's export volume of galvanized steel sheets has also increased year by year.

Galvalume-coated steel sheets have excellent corrosion resistance. The salt spray test time of ordinary pure zinc steel sheets 90/90 (units are grams/square meters) coated products is about 120 hours, but galvanized steel sheets with the same coating thickness are salt sprayed. Fog experiments can easily exceed 1,200 hours.

The reason for the huge difference in performance between galvalume (55% aluminum, remaining zinc) and pure zinc (99% zinc) in the salt spray test is that the principles of anti-corrosion are different.

Pure zinc: sacrificial protection, which preferentially corrodes the zinc layer to protect the substrate. The substrate can be well protected before the zinc layer is corroded.

Aluminum zinc: partition + sacrificial protection. The aluminum in the coating acts as a skeleton, and an aluminum oxide film is generated on the surface to isolate the corrosive medium; the zinc in the gap between the skeleton plays a sacrificial protection role when the partition fails.

The interior of an animal husbandry greenhouse (containment building) is a highly corrosive environment. Not only is the humidity high, but disinfectants of various corrosive strengths are also used. In addition, bacteria will decompose the feces of cattle, pigs or poultry into amines and acids. Among them, amines are one of the important reasons for the accelerated corrosion of aluminum-zinc coating.

Due to the presence of aluminum in the coating, the generated aluminum oxide film has very good acid resistance and can isolate the acidic corrosive atmosphere. However, the amines decomposed in the feces are very corrosive to it.

As the aluminum in the coating that acts as a barrier is gradually consumed, the zinc that acts as a sacrificial protector is exposed to the corrosive atmosphere and continues to exert sacrificial protection (protecting the steel plate by preferentially corroding itself).

Since the main component of aluminum-zinc coating is aluminum (55% aluminum), zinc is less than 45% (and a small amount of other elements), which means that in a 90g/m2 galvanized product, zinc may account for less than 40g/m2.

The galvanizing amount of 40g/m2 on one side is usually used in applications where thicker coatings are required and the corrosion conditions are not harsh, such as automobiles (paint film 80~120μm), home appliances, etc. In livestock greenhouses, this small amount of zinc is quickly consumed by the corrosive atmosphere. Subsequently, the substrate is exposed, and soon becomes perforated and rotten.