There are many processes to prevent steel corrosion, the most commonly used anti-corrosion process is in the metal surface coating (plating) non-metallic and metal protective layer, or through a special process to form a layer of chemical conversion on the surface of the protective film.

According to the substances constituting the protective layer, it can be divided into the following 3 categories.

◆ Non-metallic protective layer: organic and inorganic compounds are coated on the metal surface.

◆Metal protective layer: Coating (plating) a metal or alloy as a protective layer on the surface of iron and steel to slow down the corrosion rate. The metal or alloy used as the protective layer usually has zinc, tin, aluminum, nickel, chromium, copper and various alloys.

◆Hot-plating: The protected metal material or product is impregnated in molten metal to form a protective metal covering layer on its surface. The metal that can form a liquid state is generally a relatively low melting point, corrosion-resistant, heat-resistant metal, such as Al, Zn, etc. Chemical protective layers formed by chemical or electrochemical methods are also called chemical conversion films. According to the medium used to form the film, the chemical conversion film can be divided into oxide film, phosphate film, chromate film and so on.

Tractor contains a variety of different structural components and castings, its anti-corrosion performance is directly related to the quality of the product. Tractors and construction machinery products, the traditional anti-corrosion process of paint or plating process, but the use of such processes on the existence of "relative motion" of the parts, parts subject to extrusion and easy to bump the parts of the anti-corrosion protection effect is poor, how to solve the anti-corrosion protection of such parts, is a problem faced by many enterprises.

In the relevant data reports, the use of standard plates with different coating processes for salt spray resistance comparison of more, while the use of actual parts with different kinds of anticorrosion process for salt spray resistance comparison test reports less. This study analyzes the results of salt spray resistance tests on actual steel parts treated with several commonly used anti-corrosion processes, which is of reference significance for improving the anti-corrosion capability of products.

1, the selection of anti-corrosion treatment process

According to the general corrosion prevention requirements of machinery manufacturing enterprises, selected the commonly used 10 typical anti-corrosion treatment process for salt spray corrosion test. According to the commonly used anti-corrosion process salt spray corrosion performance indicators, the test time is set to a maximum of 600H, the test selected cold rolled plate and cold rolled parts of the same material.

Different anti-corrosion process used in the parts and corrosion test results as shown in Table 1.

A process: sandblasting - dip coating iron red epoxy primer. The cast iron plate is treated by shot blasting, the surface roughness is Ra50, and dipped and coated with iron-red epoxy primer with a viscosity of 18S produced by a company in Jiangsu, hereinafter referred to as iron-red epoxy primer.

B Technique: Shot blasting - medium temperature phosphating - dipping and coating with iron-red epoxy primer. The cast iron gasket plate is treated by shot blasting, the surface roughness is about Ra50, phosphatized with zinc at 50℃ in medium temperature, and dipped and coated with iron-red epoxy primer.

C Technique: sand blasting-phosphatization-general cathodic electrophoresis coating. The cast iron gasket plate is treated by shot blasting, the surface roughness is about Ra50, the zinc system is phosphated at 50℃ in medium temperature, and the surface is electrophoresed with general cathodic electrophoresis paint of 20μm in thickness.

D process: sandblasting-phosphating-high edge and high anti-corrosion cathodic electrophoresis coating. The cast iron gasket plate is treated by shot blasting, surface roughness Ra50 or so, phosphatized with zinc at 50℃ in medium temperature, and the surface electrophoresis is high anti-corrosive cathodic electrophoresis paint with the thickness of high edge of 20μm.

E process: sandblasting-hot-dip galvanizing. Cast iron gasket plate by shot blasting, surface roughness Ra50 or so, surface dip coating thickness of 110μm zinc layer.

Process F: Electro-galvanizing. The surface is electroplated with a zinc layer of 10μm thickness.

Process G: Spraying of acrylic top coat on castings. The surface is coated with a 20μm thick acrylic paint.

Process H: "Hot-dip zinc process [2] + topcoat spraying process" and "Electrophoretic primer + topcoat spraying process".

Process I: Cold rolled sheet - sprayed iron red epoxy primer.

Process J: Cold rolled plate - phosphating - spraying iron-red epoxy primer. The surface is first phosphated with zinc at 50℃, and then sprayed with iron-red epoxy primer.

The salt spray resistance of zinc electroplated parts is much lower than that of high margin cathodic electrophoresis corrosion protection processes (e.g., F and D processes). Different cathodic electrophoresis paints have a great influence on the corrosion and rust resistance of cast iron parts (e.g. C and D processes). General cathodic electrophoresis primer casting salt spray corrosion test 96 h appeared local rust, 240 h appeared large area rust, and the use of high edge high corrosion cathodic electrophoresis, after 600 h salt spray corrosion test, the sample surface only a small amount of "pitting"; the main reason is that the edge of the ordinary electrophoresis paint film rust resistance is poor, and the general cast iron parts of the The surface is relatively rough, the surface is equivalent to the existence of countless "edge", so for castings (especially castings with high surface roughness) must choose high edge corrosion-resistant electrophoretic coating for painting.

Phosphating process and high anti-corrosion high edge cathodic electrophoresis paint to improve the salt spray corrosion resistance of cast iron parts for the main factors affecting the composition and surface roughness of cast iron parts.

2. Comparative analysis of test results

The coating process of dipping castings with iron-red epoxy primer or spraying topcoat only has very poor corrosion and rust resistance, such as the A process and the G process. Cast iron and cold rolled plate with the same coating process, salt spray corrosion resistance is very different, cold rolled plate salt spray corrosion resistance is much better than cast iron (such as B process and J process). Using the same antirust primer, if they are not phosphating treatment (such as A process and I process), cast iron pieces of salt spray corrosion test 24 h that is a large area of corrosion, and cold rolled plate to 168 h side of the local blistering corrosion; if they are phosphating treatment (see the B process and J process), cast iron pieces of salt spray corrosion test 168 h that is, the local corrosion, while the cold rolled plate to 240 h there is no corrosion phenomenon. The phosphating process has a great influence on improving the corrosion and rust resistance of cast iron parts, and the phosphating process can improve the salt spray corrosion resistance of the parts to a greater extent (see A process and B process). Using similar coating process, cast iron parts without phosphating treatment salt spray test 24 h that large areas of corrosion, and the same parts after phosphating treatment, salt spray corrosion test to 168 h, although there is also serious corrosion, but the overall condition of the surface of the sample than the cast iron parts without phosphating treatment is much better. In the case that the coating film is not damaged by bumping, the use of high anti-corrosion high edge cathodic electrophoresis paint can meet the cast iron parts of long-term anti-rust technical requirements. For the use of relative movement between parts or bumping phenomenon, the use of "hot dip zinc technology + spray top coat technology", can effectively solve the problem of corrosion on the damaged surface of the parts coating film (such as H technology and D technology). "Hot-dip zinc technology can effectively improve the corrosion resistance of castings, and its corrosion resistance is better than that of general cathodic electrophoresis process (such as E process and C process). After hot-dip zinc technology treatment, cast iron casting parts are still free from red rust when the salt spray resistance time is 600 hours.

2.1 The impact of composition

Castings (especially cast iron parts) and general carbon structural steel (GB / T700-2006) chemical composition is different, its impurity content is carbon cast iron is mainly to determine whether the mechanical properties of the high and low to determine whether it is qualified, the national standard of its chemical properties do not have a unified requirement, but different enterprises have specified the enterprise on the chemical composition of the cast iron of the reference value. Comparison from Table 2 and Table 3 can be seen, cast iron carbon (C), sulfur (S), phosphorus (P) and other non-ferrous elements are greater than carbon steel, about 1 order of magnitude difference. The presence of these non-ferrous elements leads to iron molecules can easily form numerous primary cells with them, resulting in galvanic corrosion. Under the same temperature conditions, the speed of electrochemical corrosion is much greater than chemical corrosion.

2.2 The influence of surface roughness

The surface roughness of castings is generally in Ra12?5 above, some cast iron surface roughness up to Ra25, or even Ra50. Carbon structural steel surface roughness is generally in Ra3?2 below, the surface roughness of hot rolled is generally in Ra6?3 ~ 12?5. Roughness, on the one hand, it is easier to accelerate the adhesion of condensate and CO2, contributing to the occurrence of corrosion; on the other hand, surface roughness On the other hand, the surface roughness also affects the coverage of the surface coating protection. If only a layer of iron red epoxy primer coated on the surface of the casting, due to the leveling of the coating itself, in the casting of the rough surface of the protruding part is difficult to form an effective coating protective film. Cast iron parts treated with phosphating process, the phosphating film formed on the surface will effectively adsorb the paint and other protective substances, so that the protruding parts of the rough surface can also form a layer of phosphating film plus electrophoretic paint protective film. The surface of cast iron parts treated by phosphating process and then coated with high edge electrophoresis paint will form a multiple composite protective layer on its surface composed of phosphating film and electrophoresis coating film, so as to achieve a better protective effect.

3. Conclusion

Through the comparative study of the results of salt spray resistance testing of several steel anti-corrosion processes, it is shown that the impact of different anti-corrosion processes on the anti-corrosion performance of iron and steel parts is very large, and different anti-corrosion processes should be selected according to the use of the product's environment and the need in practice.

In the case of the same anti-corrosion performance requirements, cast iron parts need to be used than the cold rolled steel material anti-corrosion ability of stronger anti-corrosion treatment process. The use of the same anti-corrosion process for the parts, the material for the cast iron parts anti-corrosion performance is much lower than the material for the cold rolled steel material parts. For the assembly and use of products or parts that are easy to be bumped, wear and tear, the existence of relative motion, the use of general painting process or electroplating process is difficult to meet the anti-corrosion requirements, it is recommended that the use of hot-dip galvanizing or hot-dip galvanizing corrosion process, can effectively solve the problem of easy damage to the parts of the coating film corrosion. For castings (especially products with surface roughness Ra>25), the tip part of the rough surface is not easy to be coated by ordinary painting process, and the anti-corrosion effect is poor, using phosphating and high edge electrophoresis coating process, can greatly improve its salt spray corrosion resistance.