Overview of Anti-rust Coating Technology for Construction Machinery

Construction machinery refers to the machinery that serves the construction of various basic projects such as urban and rural construction, railways, highways, ports and docks, farmland water conservancy, power, metallurgy, mining, and sea and air bases. All the necessary operation machinery and equipment in the comprehensive mechanized construction of earthwork, stonework, concrete work and various building installation projects are generally referred to as construction machinery. In China, construction machinery mainly includes ten categories: excavating machinery, earth-moving and transportation machinery, engineering lifting machinery, road machinery, compaction machinery, pile driving machinery, reinforced concrete machinery, rock drilling machinery, forklifts, and special-purpose construction machinery. Its classification in other countries is largely the same. For instance, in the United States, construction machinery is called building machinery; in Japan, it is known as construction machinery; and in Russia, it is referred to as construction and road-building machinery. Common and specific construction machinery includes: hydraulic excavators, crawler bulldozers, wheel loaders, rollers, motor graders, pavers, forklifts and truck cranes, etc.

It is well known that construction machinery is an outdoor working machine that is constantly exposed to harsh environments such as the outdoor atmosphere (including Marine atmosphere, industrial atmosphere, urban atmosphere and rural atmosphere) and different water qualities (seawater, river water). At all times, it is subject to varying degrees of corrosion from different regions and environments, which has led to various "chronic diseases" and "frequently-occurring diseases" of construction machinery. Painting of construction machinery is an important link in the manufacturing process of construction machinery products. The quality of anti-rust and anti-corrosion coating is one of the important aspects of the overall quality of the product. The appearance quality of a product not only reflects its protective and decorative performance, but is also an important factor constituting the product's value.

1 Coating process

The anti-rust coating process for construction machinery can generally be divided into surface treatment process before coating and anti-rust primer coating process.

1.1 Surface treatment

Surface treatment is one of the important processes in anti-rust coating. The quality of anti-rust coating for construction machinery largely depends on the quality of surface treatment methods.

According to Imperial Chemical Company of the UK, the lifespan of coatings is restricted by three factors: surface treatment, accounting for 60%; Coating construction, accounting for 25%; The quality of the coating itself accounts for 15%.

Surface treatment methods for different components in the construction machinery industry.

Mechanical cleaning can effectively remove rust, welding slag and oxide scale from workpieces, eliminate welding stress, increase the adhesion between the anti-rust coating film and the metal substrate, and thus greatly improve the anti-rust quality of construction machinery parts. The mechanical cleaning standard requires reaching Sa2.5 level of ISO8501-1:1988. The surface roughness should reach one third of the thickness of the anti-rust coating. The steel shot used for shot blasting and shot peening shall meet the requirements of GB6484.

The surface treatment of thin plate stamping parts is generally called chemical surface treatment. The process flow is

Pre-degreasing → degreasing → hot water wash → cold water wash → acid wash → cold water wash → neutralization → cold water wash → surface adjustment → Phosphating → cold water wash → hot water wash → pure water wash → drying

The above-mentioned process can also be appropriately adjusted according to the oil and rust conditions of the thin plate stamping parts, or the acid washing process can be omitted, or the pre-degreasing process can be omitted. Degreasing and phosphating are key processes in the chemical treatment process, and these two processes directly affect the quality of the chemical treatment of the workpiece and the quality of the anti-rust coating. The process parameters and related auxiliary equipment are also factors that cannot be ignored in affecting the quality of surface treatment.

1.2 Coating process

Due to the wide range, diverse specifications, heavy overall weight and large components of construction machinery, spraying is generally adopted for coating. The spraying tools include air spray guns, high-pressure airless spray guns, air-assisted spray guns and portable electrostatic spray guns. The spraying efficiency of air spray guns is low (about 30%), and high-pressure airless spray guns waste paint. The common feature of both is that they cause relatively serious environmental pollution. Therefore, they have been and are being replaced by air-assisted spray guns and portable electrostatic spray guns. For instance, the world's leading construction machinery company, Caterpillar (CAT) of the United States, uses air-assisted spray guns for spraying and portable electrostatic spray guns for thin plate covers such as engine hoods. The coating equipment used in construction machinery generally adopts the relatively advanced water-jet spray booth. For small and medium-sized components, water curtain spray booths or pumpless spray booths can also be used. The former has advanced performance, while the latter is economical, convenient and practical. Due to the heavy weight and large heat capacity of the entire machine and its components of construction machinery, the drying of its anti-rust coating is generally carried out by using a uniform hot air convection drying method. Heat sources can be adapted to local conditions, such as steam, electricity, light diesel oil, natural gas and liquefied petroleum gas, etc.