Overhead Crane Basic: Crane Coating and Corrosion Prevention

As the important parts of overhead crane, the main beam, end beam and other important structure parts are made of steels. The steel as the most frequently used material in overhead cranes, has the features of high strength, good toughness, easy to process and cost-effective. However, its corrosion and rust resistance is poor, when used in harsh environment or conditions, which may cause potential danger to the security of the structure and overhead cranes. Therefore, the steel structures anticorrosion treatments has a high necessity.

Overhead crane after coating 

Causes to steel corrosion

To solve the problem of rust of the steel structure, the first thing to understand the causes of steel corrosion. Generally, there are two reasons that cause the corrosion of the steel materials.

• The corrosion mechanism of steel in the room temperature (below 100 ℃)

The corrosion of steel is mainly electrochemical corrosion in the room temperature，the steel is corroded by the effect from moisture, oxygen and other contaminant etc .Under the room temperature, critical humidity of the steel is 60% ~ 70%.

• The corrosion mechanism of steel in the high temperature (above 100 ℃)

The corrosion of steel is mainly chemical corrosion in the high temperature. Under the high temperature condition, the steel and the dry air such as O2, H2S, SO and Cl2, etc., contact with each other and generate the corresponding compound, such as chloride, sulfide and oxide, which makes the corrosion to the steel.

Steel structure corrosion prevention

Based on the electrochemical principle of steel corrosion, to coat the corrosion prevention layer to the steel surface is the most commonly used method. Commonly, there are three types of corrosion prevention layers:

• Metal coating: A layer of metal is coated on the steel surface by electroplating, plating, electroless plating, hot dip, etc.
• Chemical protective layer: A layer of anti-corrosion chemical compound film is coated on the surface of the steel by chemical or electrochemical methods.
• Non-metallic protective layer: A layer of coating is coated on the surface of the steel by shelling-out and spraying to protect the steel, which is the most frequently used method.

Anticorrosive coating

Anticorrosive coating is the most commonly used method to protect the steel surface at present. The anticorrosive coating can be classified into three layers, the primer coat, intermediate coat, and finishing coat, each of which has its own specific features and functions. The three layers together form a composite coating to protect the steel from the corrosion, and prolong the working life.

• Primer coat: Primer coat is commonly coated by the paint which is zinc rich primer and epoxy iron red primer.
• Intermediate coat: Intermediate coat is painted by epoxy micaceous iron oxide and glass flake epoxy paint.
• Finishing coat: Finishing coat mainly uses the epoxy paint and chlorinated rubber coat and polyurethane coat.

Anticorrosive coating selection

Confronted with various types of coatings and painting, how to make the right choice? The following tips are for your reference:

• The local climate and environment of the application.
• The ease of the construction conditions.
• The correction usage of the layers of anticorrosive coatings.
• The economy, durability and toxicity of the painting, etc.

Thickness of the coating

On selecting the right coating, the thickness of the coating also matters.If the coating is too thin or uneven, the coating will have pinhole, from which the steel will get corrosion, and become into rusty bulge which will expand gradually, and finally the coating will drop off. If the coating is too thick, it may enhance the protection but also will reduce adhesion and mechanical properties, and increase the costs.

General requirements of the coating thickness

• For the components which has common requirements to the coating, it usually needs two primer and two fish coat when manual and powered dedusting is used.
• For the components which has higher coating requirements, it would be better that the primer should be painted two times, the intermediate lacquer 1 to 2 times, and the finish coat 2 time when a jet blasting is used.
• The total dry film coating thickness is not less than 120μm (weak erosion) and 150μm (moderate erosion), 200μm (an important component of a strong erosion). The thickness of the film should be even, continues, no pinhole in order to achieve the good anti-corrosion effect.