1、 Construction preparation

 　　1Material Science

 　　1）The selection of anti-corrosion materials for steel structure engineering shall meet the design requirements.Anticorrosive materials include primer, topcoat and thinner.The anti-corrosion primer of building steel structure engineering includes red lead oil-based antirust paint, molybdenum chromium red epoxy ester antirust paint, etc.; the anti-corrosion top coat of building steel structure includes alkyd enamel of various colors and alkyd mixed paint of various colors.All kinds of anti-corrosion materials shall comply with the provisions of relevant national technical indicators, and shall also have the product ex factory certificate.

 　　2Main machines and tools:

 Sandblasting gun, air pump, recovery device, spray gun, spray paint air pump, hose, blade, hand wheel, abrasive cloth, wire brush, cotton wire, small compressor, paint bucket, brush, pickling tank and accessories.

 　　3Operating conditions:

 　　1）Painters shall have operation certificates for special types of work.

 　　2）The steel structure engineering has been inspected and accepted before the anti-corrosion coating project, and meets the design requirements. 

Anti corrosion and rust removal of steel structure engineering

　　3）The anti-corrosion coating operation site should have safety protection measures, fire prevention and ventilation measures to prevent fire and personnel poisoning accidents.

 　　4）Appropriate weather should be selected for outdoor anti-corrosion construction, and no operation should be carried out in case of strong wind, rain and severe cold.

 2、 Operation technology

 　　1technological process:

 Base surface cleaning → primer coating → finish coating → inspection and acceptance

 　　2Base surface cleaning:

 　　1）The painting of building steel structure engineering shall be carried out after the steel structure installation acceptance is qualified.Before painting, the rust, weld coating, welding spatter, oil stain, dust and other debris on the parts to be coated shall be cleaned up.

 　　2）The quality of base surface cleaning and derusting is directly related to the quality of coating.Therefore, the base surface rust removal quality of the coating process is divided into grade I and II, as specified in the table below.

 Derusting quality grade of steel structure

 Derusting method of grade quality standard

 　　1Metal color is exposed on the steel surface, sandblasting, shot blasting and pickling

 　　2It is allowed to leave clean rolling skin on the steel surface, and general tools (wire brush and abrasive cloth) shall be removed

 　　3）In order to ensure the coating quality, the following derusting processes can be selected according to different needs.

 Sand blasting is a kind of rust removal method that uses the pressure of compressed air to continuously impact the surface of steel members with quartz sand or iron sand, and clean up the rust, oil and other impurities on the steel surface, and expose the natural color of metal steel.This method has the advantages of high efficiency and thorough derusting.

 Acid pickling is to dip the steel members to be coated in the acid pool, and remove the oil and rust on the surface of the components with acid.The pickling process has high efficiency and thorough rust removal. However, the components must be washed with hot water or clean water after pickling. If there is residual acid, the corrosion of components will be more severe.

 Manual derusting refers to the manual use of some relatively simple tools, such as scraper, grinding wheel, abrasive cloth, wire brush and other tools to remove rust on steel members.This method has low efficiency, poor working conditions and incomplete rust removal.

 　　3Primer coating:

 　　1）Mix red lead antirust paint, control the viscosity, consistency and thinness of the paint, and fully stir when cashing to make the color and viscosity of the paint uniform.

 　　2）When brushing the first layer of primer, the brushing direction should be consistent and the joints should be neat.

 　　3）The principle of frequent touch and short brush should be adopted to prevent the brush from falling due to too much paint. 

Anti corrosion coating of steel structure

　　4）After the first painting, a certain time interval should be kept to prevent the second time before the first time is dry, which will cause the paint to flow, wrinkle and quality.

 　　5）After the first time is dry, brush it for the second time. The direction of the second coat should be perpendicular to the direction of the first coat, which will make the thickness of the paint film uniform.

 　　6）It takes at least 4-8h after the primer coating to reach the surface drying, and the topcoat should not be applied before the surface drying.

 　　4Finish painting:

 　　1）The interval time between primer and topcoat is generally long.The steel members shall be sent to the construction site for assembly after being coated with antirust paint, and the top coat shall be applied uniformly after the assembly.In this way, it is necessary to clean the surface of the steel structure, remove the welding flux of the installation weld, and make up the paint in advance for the components that burn off or touch off the paint.

 　　2）The top coat with the same color shall be selected for the preparation of the top coat. The prepared thinner shall be suitable. The top coat shall be fully stirred before use to keep the color uniform.Its working viscosity and consistency should ensure that the coating does not flow down and show brush marks.

 　　3）The top coat should be continuously mixed in the process of use, and the brushing method and direction are the same as the above process.

 　　4）The nozzle diameter and spraying pressure should be adjusted properly when spraying is adopted in the coating process. The hose of spray gun can be stretched to the operation area freely. The air pressure of air compressor should be 0.4 ~ 0.7n/m2

 　　5）During spraying, the distance between the nozzle and the coating should be maintained. Generally, the distance between the spray gun and the working surface should be about 100 mm. The angle between the spray gun and the steel structure base surface should be kept vertical, or the nozzle should be slightly inclined upward.

 　　6）When spraying, the nozzle should move in parallel, move steadily, with the same speed, and keep the coating uniform.However, when spraying, the thickness of the coating is generally thin, so it should be sprayed several times. Each layer should be sprayed when the upper layer of paint film has been dried.

 　　5Coating inspection and acceptance:

 　　1）During and after surface coating construction, the coated workpiece shall be protected to prevent dust and other debris.

 　　2）After coating treatment inspection, the coating color should be consistent, bright color, no wrinkle skin, no pimple.

 　　3）The thickness of coating film is measured by contact type paint film thickness meter. Generally, the thickness of three points is measured by paint film thickness gauge, and the average value is taken.

 3、 Quality standard

 　　1The guarantee items shall meet the following requirements:

 　　1）The varieties, models and quality of coatings, diluents and curing agents shall meet the design requirements and the provisions of current national standards.

 Test method: check the quality certificate or re inspection report.

 　　2）The rust removal on the steel surface before coating shall meet the design requirements and the provisions of the current national standards: the steel surface after chemical rust removal shall show the metal color.

 The treated steel surface shall be free of welding slag, welding scar, dust, oil stain, water and burr.

 Inspection method: check with scraper and observe with pictures specified in current national standard “rust grades and derusting grades of steel surface before coating”.

 　　3）The coating shall be free from peeling and rust.

 Test method: observation.

 　　2Basic items shall meet the following requirements:

 　　1）Appearance quality of painting works:

 Qualified: the painting should be uniform, without obvious wrinkle and bubble, with good adhesion.

 Good: the painting should be uniform, the color is consistent, no wrinkle, sagging and bubbles, good adhesion, clear and neat color separation line.

 Test method: observation.

 　　2）Quality of component repainting:

 Qualified: the paint film should be complete.

 Excellent: according to the coating process layer repair brush, complete paint film, good adhesion.

 Inspection quantity: spot check 10% according to the number of each type of component, but no less than 3 pieces.

 Test method: observation.

 　　3）The allowable deviation items and inspection methods of dry paint film thickness of coating engineering shall comply with the provisions in table 5-26.The required thickness value and allowable deviation value of dry paint film shall comply with the provisions of code for construction and acceptance of steel structure engineering.

 Inspection quantity: spot check 10% according to the number of similar components, but it should not be less than 3 pieces. Each piece should be measured at 5 places. The value of each place is the average value of dry paint film thickness of 3 measuring points with a distance of about 50 mm.

 　　4）Finished product protection

 　　1After painting, steel members shall be temporarily enclosed and isolated to prevent stepping on and damaging the coating.

 　　2After coating, steel members shall be covered in case of strong wind or rain within 4 hours to prevent dust and moisture from affecting the adhesion of coating.

 　　3When the coated components need to be transported, attention should be paid to prevent bumping, dragging on the ground and coating damage.

 　　4The coated steel members shall not contact with acid liquid to prevent the coating from biting.

 4、 Quality problems that should be paid attention to

 　　1The temperature of coating operation should be between 5 ℃ and 38 ℃. When the weather temperature is lower than 5 ℃, the corresponding low-temperature coating materials should be used.

 　　2When the temperature is higher than 40 ℃, the coating operation shall be stopped.When the component temperature exceeds 40 ℃, air bubbles will be produced when the steel surface is coated with paint, which will reduce the adhesion of paint film.

 　　3When the air humidity is more than 85%, or there is condensation on the surface of components, coating operation is not suitable.

 　　4Before the fabrication of steel members, the concealed parts of components and the parts of structural interlayer that are difficult to be derusted shall be derusted and painted in advance.

 5、 Whether the steel structure coating project is accepted or not is related to the long-term service life of the steel structure, so as to avoid corrosion and exposure of some parts, which may cause hidden danger to the overall quality of the steel structure engineering. The author thinks that the inspection of steel structure coating engineering can be divided into two parts: appearance inspection and thickness inspection.

 　　1. Standard of coating appearance:

 The appearance of the coating shall be uniform, flat, plump and glossy: its color shall be consistent with the color specified in the design; defects such as undercutting, crack, peeling, pinhole and other defects are not allowed; the coating installed slowly on steel structure is allowed to have slight sagging, brush marks, wrinkling and a small amount of particle dust which will not affect the protection performance.

 　　2.Standard of coating thickness: the thickness of coating can be measured by paint film thickness meter, and the total thickness must meet the standard specified in the design,

 　　(1) Sample size of thickness measurement:

 20% of the main components such as trusses and beams, 10% of the secondary components, three parts of each component, and three places of every 10 m2 of plates, beams and box girders.

 　　(2) Provisions for inspection points:

 For beams or members with a width of less than 150 mm, three points shall be measured at each point, and the point position shall be perpendicular to the side length. The distance between the points shall be 1 / 4 of the width of structural members. For beams or members with a width of more than 150 mm, 5 points shall be measured at each place. The center position of the points is not limited. However, the edge points shall be more than 2 mm away from the edge of the component. The five detection points shall be the intersection of four corners of a 100 mm square square square and the square diagonal line.

 　　(3) If the total average thickness of the coating inspection side reaches 90% of the specified thickness, it is qualified.When calculating the average value, the measuring points exceeding 20% of the specified thickness shall be calculated as 120% of the specified thickness.