Analysis and preventive measures of white rust and red rust on the surface of galvanized steel sheets

White rust and red rust are common defects in galvanized steel sheets. Different thicknesses of galvanized layers will cause harm to the generation time of white rust and red rust?

Let us prove through experiments that the thickness of the galvanized layer of the material has no harm to the generation time of white rust on the surface of the galvanized steel sheet; the thickness of the galvanized layer has an impact on the generation time of red rust on the surface of the galvanized steel sheet. The thicker the zinc layer, the longer the incubation period for red rust. So how do white rust and red rust occur on the surface of galvanized steel sheets?

Zinc is an extremely active metal material. The surface of zinc will undergo chemical changes with the water vapor in the air to form multi-layer porous and sticky Zn(OH)2. Then Zn(OH)2 will further chemically react with carbon dioxide in the atmosphere to form a thin, high-density, certain adhesive layer of basic zinc carbonate (2ZnCO3.3Zn(OH)2), which can also prevent The zinc layer corrodes further. When galvanized steel sheets are closely packed and placed in a humid air, there is no free flowing air on the surface between the galvanized steel sheets, and the local surface of the galvanized layer will not be able to form the above-mentioned corrosion product protection. The chemical change of the film will instead cause electrochemical corrosion and produce white rust. The production mechanism of white rust is actually the principle of "oxygen concentration difference corrosion battery".

The severity of white rust mainly depends on the composition of the condensation water between the coatings and the duration of the environment. If the condensed water contains chloride from seawater, it will increase the conductivity of the water, thereby increasing the rate of electrochemical corrosion and accelerating the corrosion rate of the galvanized layer.

The root cause of red rust

After the zinc layer on the surface of the galvanized steel sheet is corroded away, the substrate begins to react electrochemically with the moisture in the air. The main chemical components in the substrate are iron and carbon. Iron and carbon create a galvanic cell and a galvanic cell reaction occurs. The reaction equation is as follows:

4Fe+6H2O+3O2=4(FeOH)3

2(FeOH)3=Fe2O3+3H2O

Fe2O3 is red and is the main component of rust.

The above-mentioned proof materials show that white rust is basically produced at the same time during the test and inspection of 80g/m2 galvanized layer and 120g/m2 galvanized layer. The unpainted galvanized steel sheet with a zinc layer of 80g/m2 will produce red rust later.

What to do if the galvanized sheet turns white

Check whether the surface layer of the hot-dip galvanized product turns white after being wetted. It is a normal phenomenon that the zinc on the surface layer is oxidized. Passivation treatment is carried out after electro-galvanizing. Generally, there will be no white spot problem. If the requirements for corrosion resistance are higher, you can also consider adding a post-sealing agent.

1. It is a quality problem of passivation. The most common situation is that the passivation is not uniform enough or the thickness does not meet the standard.

2. There is residual moisture on the surface of the steel that has not been treated. When curling, the surface of the steel contains moisture that has not been dried, or it has not been thoroughly dried during passivation.

3. The transportation conditions are poor.

Specifically in:

1. Get damp or wet by rain during storage and transportation.

2. The finished steel products are stored for too long.

Therefore, we can know that the anti-corrosion work of galvanized steel sheets should be done from two aspects:

1. It is necessary to use passivation products with better performance to passivate galvanized steel sheets.

2. It is necessary to continuously improve the storage conditions of galvanized steel sheets.

Causes of white rust

①The indoor air humidity is high.

② After the galvanized sheet is produced and processed, the temperature of the steel coil is too high, causing a layer of water film to condense on the surface. When the moisture content in the air reaches 60% or in

In the range of 85~95% and PH<6, the corrosion reaction is more severe. When the water temperature reaches about 70°C, it corrodes the zinc layer faster.

③ Corroded by acidic gas or liquid.

④ The finished product is stored in the warehouse without being dried and processed correctly in time when packaging.

Elimination strategy
Prevent water leakage during packaging or transportation and water seepage from steel coils;
Control the ambient temperature of the coiling and packaging areas to be greater than the dew point temperature;
Strictly abide by packaging temperature and time regulations;
Steel coils with water seepage should be dealt with in time.