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　　Container pretreatment is one of the key technologies in container manufacturing and the first step in container manufacturing. Pretreatment quality is directly related to the manufacturing quality and service life of the container. Therefore, all the container owners have written the requirements of surface treatment cleanliness level and roughness into the technical specifications of container manufacturing.

　　1、 Investigation of steel quality before shot blasting

　　Before shot blasting, in order to make the quality of the pre-treatment process meet the requirements of the specified grade, the quality of the steel which has not entered the pre-treatment process is investigated. The items to be investigated include: steel type, material, number, manufacturer, relevant clauses in the quality assurance certificate and technical specification, check one by one and register. In addition, the surface corrosion of steel plate before coating should be investigated. According to ISO 8501-1 and GB 50205-1992, the corrosion degree of steel plate surface is divided into four grades: A, B, C and D. Evaluate the grade by visual comparison according to standard samples and photos. Grade D steel plate is the steel plate that can not be used in container manufacturing, and grade C steel plate is not allowed to be used in door plate, side plate, front plate and other parts.

　　In order to improve the film quality of shot blasting and shop primer, some auxiliary measures can be taken before shot blasting, for example, the parts with severe rust spots can be pre coated with wire brush; the steel surface, especially the section steel, shall be scrubbed with xylene and other solvents to remove oil stains.

　　In addition, the soot blowing system and the sand dust separation system in the shot blasting machine shall work normally; the steel sand (abrasive) itself shall be free of oil, ash and agglomeration.

　　2、 Steel surface cleanliness after shot blasting

　　In the technical documents prepared by the box owner, it is specified that the cleanliness of steel surface treatment must meet the SA 2.5 standard of ISO 8501-1, or SIS 05590-1967 SA 2.5 standard, or sspc-sp10 is close to the white level.

　　3、 Roughness after shot blasting

　　In the technical documents of most container owners, the roughness of the steel surface of the container after shot blasting is Rz35-50um, that is to say, the average of the peak and valley height of the roughness is 35-50um. In some foreign technical documents, RA roughness parameter may be quoted, which is the average distance between the assumed center line between the peak and valley values of the value. Theoretically, the roughness of sand blasting (shot blasting) RZ is 4-6 times of RA.

　　There are many methods to determine the surface roughness value. The international organization for Standardization (ISO / TC35 / SC12) has specially formulated the international standard ISO 8503. It has specified the specific technical requirements and definitions of the grading method, the micro adjustment method and the contact method of the comparative sample block. In the container coating process, the most commonly used is the surface roughness comparison template. The sample plate is divided into two parts, which are indicated by the letters "s" and "C" respectively, to evaluate the surface roughness of the steel plate after shot blasting and abrasive blast cleaning. Each comparison plate includes 4 parts representing different roughness, and the nominal roughness values of each part are shown in the table below.

　　In the actual assessment of roughness, the surface of the steel to be tested shall be removed first to remove the ash and debris; then, according to the type of abrasive, select a suitable comparison sample block (g or s sample block) to form a comparison with a certain area of the surface to be tested, and then compare the surface to be tested with four parts of the sample block by visual inspection in order to determine the level and approximate value of the surface roughness to be measured.

　　The above content is reproduced in "anticorrosive coating and coating" collected by Mr. Li Minfeng