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Rust grades and preparation grades of steel surfaces before application of paints and related products GB8923-88 Rust grades and preparation grades of steel surfaces before application of paints and related products This standard specifies the visual assessment grades of rust grades and rust removal quality on steel surfaces before application. It is suitable for use on hot-rolled steel surfaces treated by spray or blast removal, hand and power tool removal, and flame removal. The evaluation of the surface rust removal grade of cold-rolled steel can also be used as a reference. This standard is equivalent to the international standard ISO8501-1: 1988 "Preparation of steel before application of paints and related products—Visual assessment of surface cleanliness—Part 1: Corrosion grade and rust removal grade of uncoated steel and steel after complete removal of the original coating. 1. General Principles 1.1 This standard divides the original rust degree of the uncoated steel surface into four "rust levels", and divides the quality of the uncoated steel surface and the steel surface after the original coating has been completely removed into several "rust removal levels". The corrosion grade and rust removal grade on the steel surface are determined by text description and photos of typical samples. 1.2 This standard expresses the corrosion level and rust removal level based on the visual appearance of the steel surface. When assessing these levels, direct observation should be made with normal vision under moderate lighting conditions without the aid of magnifying glasses and other equipment. 2. Corrosion grade The four corrosion grades on the steel surface are represented by A, B, C and D respectively. Photos of typical samples of these rust levels are found in Chapter 5 and are described below.: A A steel surface that is fully covered with oxide scale and has almost no rust ； B The surface of steel that has been corroded and part of the oxide scale has peeled off ； C The oxide scale has peeled off due to rust, or can be scraped off, and there is a small amount of pitting on the steel surface ； D The oxide scale has been completely peeled off due to rust, and the surface of the steel has been generally pitted. 3. Rust removal grade 3.1 General 3.1.1 The rust removal grade on the steel surface is represented by the letters "Sa", "St" or "FI" representing the rust removal method used. If there is an Arabic numeral after the letter, it indicates the degree of removal of scale, rust, paint coating and other attachments. 3.1.2 For text descriptions of steel surface rust removal grades, see 3.2, 3.3 and 3.4. See Chapter 5 for photos of typical samples of these grades. Note: (1) In the definition of each rust removal grade in this chapter, the term "attachments" can include welding slag, welding spatter, soluble salts, etc. (2) In this chapter, "when oxide scale. When "rust or paint coating" can be peeled off the steel surface with a metal putty scraper, it should be considered as weakly adhered. 3.2 Spray or projectile rust removal 3.2.1 Spray or projectile rust removal is represented by the letter “Sa”. 3.2.2 Before spraying or projecting rust removal, thick rust layers should be removed. Visible grease and dirt should also be removed. After blasting or blasting rust removal, the steel surface should be cleared of floating dust and debris. 3.2.3 For steel surfaces that have been rust-removed by spraying or blasting, this standard stipulates four rust removal levels. Its text description is as follows: Sa1 Mild spraying or blasting rust removal The steel surface should be free of visible grease and dirt, and there should be no attachments such as scale, rust, and paint coatings that are not firmly attached. See photos BSa1.CSal and DSa1.Sa2. Thorough blasting or blasting of rust-removed steel surfaces should be free of visible grease and dirt, and attachments such as scale, rust, and paint coatings have been basically removed, and their residues should be firmly adhered. See photos BSa2, CSa2 and DS2. Sa21/2 Very thorough blasting or blasting rust removal. The steel surface should be free of visible grease, dirt, scale, rust and paint coatings and other attachments. Any remaining traces should be only dots or stripes of slight color spots. See photos ASa21/2, BSa1/2, CSa1/2, and DSa21/2. Sa3 To make the steel appear clean, the surface of the steel should be free of visible grease, dirt, scale, rust, paint coatings and other attachments, and the surface should show a uniform metallic color. See photos ASa3, BSa3, CSa3 and DSa3 3.3 Rust removal with hand and power tools 3.3.1 Rust removal with hand and power tools such as scrapers, hand or power wire brushes, power sandpaper discs or grinding wheels, indicated by the letter "St". 3.3.2 Before removing rust from hand and power tools, thick rust layers should be removed, and visible grease and dirt should also be removed. After hand and power tool rust removal, the steel surface should be cleaned of loose dust and debris. 3.3.3 For steel surfaces that have been derusted by hand and power tools, this standard has two derusting levels, which are described in the following text:: St2 Thorough hand and power tool derusting The steel surface should be free of visible grease and dirt, and free of loose scale, rust and paint coatings, see photos BSt2, CSt2 and DSt2. St3 Very thorough hand and power tool rust removal. Steel surfaces should be free of visible grease and dirt, and free of loose scale, rust and paint coatings. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster. See photos BSt3, CSt3 and DSt3. 3.4 Flame rust removal 3.4.1 Flame rust removal is represented by the letters “FI”. 3.4.2 Before flame rust removal, thick rust layers should be removed. Flame rust removal should include using power wire brushing to remove the products attached to the surface of the steel after the flame heating operation. 3.4.3 The text description of the rust removal grade after flame rust removal is as follows:: The surface of FI flame derusted steel should be free of scale, rust, paint coating and other attachments, and any remaining traces should only be surface discoloration (dark shadows of different colors). See photos AFI, BFI, CFI and DFI. 4. Visual assessment of steel surface corrosion grade and rust removal grade 4.1 Assessment of steel surface corrosion grade and rust removal grade should be carried out under good scattered sunlight or artificial lighting conditions with comparable illumination. Examiners should have normal vision. 4.2 The surface of the steel to be inspected shall be compared visually with the corresponding photographs (see Chapter 5). Photos should be taken close to the steel surface. 4.3 When assessing the rust level, the rust level indicated in the photo of the corresponding more severe rust level shall be used as the assessment result. ； When evaluating the rust removal grade, the rust removal grade indicated by the photo that is closest to the surface appearance of the steel shall be used as the evaluation result. Note: (1) There are many factors that affect the visual assessment results of steel surface rust removal grade, among which the main ones are: a. Abrasives used for spray or projectile rust removal, tools used for hand and power tool rust removal ； b. Surface corrosion status of steel materials that do not belong to the standard corrosion grade ； c. The color of the steel itself ； d. Differences in roughness of various parts due to different degrees of corrosion ； e. The surface is uneven, such as depressions ； f. Tool scratches ； g. Uneven lighting ； h. When spraying or projecting rust removal, shadows are caused by the different angles at which the abrasive impacts the surface. ； i. Abrasive embedded in the surface. (2) To visually assess the rust grade of the originally painted steel surface, only photos with rust grade symbols C and D (such as DSt21/2 and CSt21/2) can be used. Which photo to choose (for example, DSt21/2 or CSt21/2) depends on the degree of pitting corrosion on the steel surface (see Chapter 2). 5. Photos 5.1 This standard includes 28 typical sample photos of steel surface corrosion levels, which are consistent with the international standard ISO 8501-1: The same photo from 1988. In case of any dispute, the photos in ISO 8501-1 shall be the basis for arbitration. 5.1.1 There are 4 photos indicating the rust level, marked A, B, C and D respectively. 5.1.2 There are 24 photos showing the level of rust removal achieved by spray or projection rust removal, hand and power tool rust removal, and flame rust removal. These photos are marked with symbols of the original rust grade before rust removal and the rust removal grade after rust removal, such as BSa21/2. 5.2 The 14 photos of spray or projection rust removal represent the surface condition of the steel after dry spray rust removal using quartz sand abrasives. When other types of abrasives are used for spray or projection rust removal, the steel surface after rust removal may have different tones. 5.3 This standard does not contain photos of ASa1, ASa2, ASt2 and Ast3. 5.4 Typical sample photos of rust levels and rust removal levels are arranged in the order shown in the figure below and are an integral part of this standard.

One additional point: "Steel surface corrosion grade and rust removal grade before painting" (GB 8923-88). The standard divides the surface conditions of steel materials before treatment into four levels: A, B, C, and D. Sand blasting or shot blasting rust removal is represented by the symbol Sa and is divided into four levels, Sa1, Sa2, Sa21/2, and Sa3. Different steel surfaces have different requirements. Anti-corrosion coatings generally require no less than Sa2, and high-grade coatings generally require no less than Sa21/2. Manual and power rust removal, represented by the symbol St, is divided into two levels: St2 and St3. The paint quality of manual rust removal is much lower than that of sandblasting. The standard also stipulates flame rust removal, but it is not widely used in China. There are no provisions for pickling and rust removal. For some steel components with complex or large shapes, pickling and phosphating are used to remove rust in China, and there is some experience in using them. The coating requirements for the surface of cement mortar or concrete should mainly have a certain strength, no floating ash, oil stains, etc., and the moisture content of the base surface should not be greater than 6%. Because a large amount of calcium hydroxide precipitates when cement hardens, it will react with dry oil-based paints and destroy their bonding force. Therefore, the cement surface should be neutralized, but there is no such requirement for resin-based paints.

"Steel surface corrosion grade and rust removal grade before painting" (GB 8923-88) has specific regulations: 3. Rust removal grade 3.1 General 3.1.1 The steel surface rust removal grade is represented by the letters "Sa", "St" or "F1" representing the rust removal method used. If there are Arabic numerals after the letters, they indicate the degree of removal of scale, rust, paint coatings and other attachments. 3.1.2 For text descriptions of steel surface rust removal grades, see 3.2, 3.3 and 3.4. See Chapter 5 for photos of typical samples of these grades. Note: (1) In the definition of each rust removal grade in this chapter, the term "attachments" can include welding slag, welding spatter, soluble salts, etc. (2) In this chapter, when oxide scale, rust or paint coating can be peeled off from the steel surface with a metal putty scraper, it shall be regarded as weak adhesion. 3.2 Spray or projectile rust removal. 3.2.1 Spray or projectile rust removal is represented by the letter “Sa”. 3.2.2 Before spraying or projectile rust removal, thick rust layers should be removed. Visible grease and dirt should also be removed. After blasting or blasting rust removal, the surface of the steel should be cleared of floating dust and debris. 3.2.3 For steel surfaces that have been rust-removed by spraying or blasting, this standard stipulates four rust removal levels. The text description is as follows: Sal Mild spray or projection rust removal The steel surface should be free of visible grease and dirt, and there should be no attachments such as scale, rust, and paint coatings that are not firmly attached. See photos BSal, CSal and DSal. Sa2 Thorough spraying or blasting of rust-removed steel surfaces will result in no visible grease and dirt, and attachments such as scale, rust, and paint coatings have been basically removed, and their residues should be firmly attached. See photos BSa2, CSa2, DSa2. Sa2 * 0.5 Very thorough spraying or blasting rust removal will leave the steel surface free of visible grease, dirt, scale, rust, paint coatings and other attachments. Any remaining traces should be only dots or stripes of slight stains. See photo ASa2 * 0.5, BSa2 * 0.5, CSa2 * 0.5, and DSa2 * 0.5 Sa3 The steel surface should be free of visible grease, dirt, scale, rust, paint coating and other attachments to make the steel appear clean. The surface should show a uniform metallic color. See Photos ASa3, BSa3, CSa3 and DSa3 3.3 Hand and Power Tool Rust Removal 3.3.1 Rust removal using hand and power tools, such as scrapers, hand or power wire brushes, power sandpaper discs or grinding wheels, is represented by the letter "St". 3.3.2 Before derusting hand and power tools, thick rust layers should be removed, and visible grease and dirt should also be removed. After hand and power tool rust removal, the steel surface should be cleaned of loose dust and debris. 3.3.3 For steel surfaces that have been rusted by hand and power tools, this standard stipulates two rust removal levels, and the text description is as follows: St2 Thorough manual and power tool rust removal of steel surfaces should be free of visible grease and dirt, and free of loose scale, rust, paint coatings and other attachments. See photos BSt2, CSt2, and DSt2. St3 Very thorough hand and power tool rust removal. Steel surfaces should be free of visible grease and dirt, and free of loose scale, rust and paint coatings. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster. See photos BSt3, CSt3 and DSt3. 3.4 Flame rust removal 3.4.1 Flame rust removal is represented by the letters "FI". 3.4.2 Before flame rust removal, thick rust layers should be removed. Flame rust removal should include using power wire brushing to remove the products attached to the surface of the steel after the flame heating operation. 3.4.3 The textual description of the rust removal grade after flame rust removal is as follows: FI The surface of flame rust removed steel should be free of scale, rust, paint coating and other attachments, and any remaining traces should only be surface discoloration (shadows of different colors). See photos AFI, BFI, CFI, and DFI.

Classification of rust removal grades for steel structures 1. The standard for steel surface corrosion and rust removal grade is:* * Standard GB8923-88 "Steel surface corrosion grade and rust removal grade before painting". 2. The standard divides rust removal levels into three types: spray or projection rust removal, manual and electric rust removal, and flame rust removal. 3. Spray and projectile rust removal, represented by the letter "sa", is divided into four levels: 1. The standard for steel surface corrosion and rust removal grades is * * Standard GB8923-88 "Steel surface corrosion grade and rust removal grade before painting". 2. The standard divides rust removal levels into three types: spray or projection rust removal, manual and electric rust removal, and flame rust removal. 3. Spray and projectile rust removal, represented by the letter "sa", is divided into four levels: sa1 - mild blasting and rust removal. There is no visible grease, dirt, unattached oxide scale, rust, paint coating and other attachments on the surface of the steel. sa2 - Thorough spray or blast rust removal. There is no visible grease, dirt, and attachments such as scale, rust, etc. on the steel surface. sa21/2 - Very thorough spray or blast rust removal. There is no visible grease, dirt, oxide scale, rust, paint coating and other attachments on the surface of the steel. Any remaining traces are only dots or strips of slight stains. sa3 - spray or blast rust removal to make the steel surface very clean. There is no visible grease, dirt, oxide scale, rust, paint coating and other attachments on the steel surface. The surface shows a uniform metallic color. Manual rust removal grade: St2 Thorough hand and power tool rust removal Steel surfaces should be free of visible grease and dirt, and free of loose scale, rust and paint coatings. St3 Very thorough hand and power tool rust removal. Steel surfaces should be free of visible grease and dirt, and free of weakly adherent scale, rust and paint coatings. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster.

When the once-through boiler is ignited, the water in the evaporation heating surface is forced to flow driven by the pump. As the heat load gradually increases, the water temperature continues to rise. Once it reaches the saturation temperature, the water begins to vaporize, and the specific volume of the working fluid increases significantly. At this time, the working fluid in the tube after the vaporization point will be pushed to the boiler outlet, so that the volumetric flow rate of the working fluid entering the starting separator is significantly greater than the volumetric flow rate at the boiler inlet. This phenomenon is called expansion. The basic reason for the expansion phenomenon is that the specific volume difference between steam and water is too large. When starting, all the water flowing through the evaporation heating surface is water. During the heating process, the water temperature gradually increases. The working fluid at the intermediate point first reaches the saturation temperature and begins to vaporize. The volume suddenly increases, causing the local pressure to rise and violently pushing the working fluid behind it to the outlet, resulting in a large instantaneous discharge of the working fluid at the boiler outlet. During startup, excessive expansion will make it difficult to control the working fluid pressure in the boiler and the water level of the starting separator. The main factors affecting the amount of expansion are: (1) The position of starting the separator. The closer the starting separator is to the outlet, the more water is stored in the heating surface between the vaporization point and the separator, the greater the expansion during vaporization, and the longer the expansion lasts. (2) Starting pressure The lower the starting pressure, the lower the saturation temperature. The vaporization point of water moves forward, resulting in a large amount of water stored in the heating surface behind the vaporization point, and a large difference in specific volume between steam and water, thus increasing the amount of expansion. (3) Feed water temperature The feed water temperature affects the sooner or later the working fluid starts to vaporize. The feed water temperature is high, the vaporization point is advanced, and the water storage volume in the heating surface behind the vaporization point is large, which increases the expansion amount. (4) Fuel input speed The fuel input speed is the combustion rate at startup. The combustion rate is high, the heat load in the furnace is high, the temperature of the working fluid rises quickly, the vaporization point is advanced, and the expansion is increased.

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* * Standard GB8923-88 "Steel surface modification grade and rust removal grade before painting" divides the rust removal grade into three types: spray or projectile rust removal, manual and power rust removal, and flame rust removal.: (1) Spray or projectile rust removal is represented by the letter "Sa". divided into four levels: Sa1 Light blasting or blasting for rust removal. There is no visible grease or dirt on the surface of the steel, and there are no attachments such as scale, rust, and paint coatings that are not firmly attached. Sa2 Thorough spray or blast removal of rust. There should be no visible grease, dirt, scale, rust and other attachments on the steel surface, and the residues should be firmly attached. The projection dents formed after shot blasting are evenly arranged on the steel surface, and the anti-slip coefficient reaches 0.35~0.45 Sa2.5. Very thorough spray or projection rust removal. The steel surface should be free of visible grease, dirt, scale, rust, paint coatings and other attachments, and any remaining traces should be only dots or strips of slight stains. The projection dents formed after shot blasting are evenly arranged on the surface of the steel, and the anti-slip coefficient reaches 0.45~0.5 Sa3, making the steel surface clean by spray or projection rust removal. The steel surface should be free of visible grease, dirt, scale, rust, paint and other attachments, and the surface should show a uniform metallic luster. (2) Manual and power rust removal: Indicated by the letters "St" and divided into two levels: St2 Thorough hand and power rust removal. There is no visible grease and dirt on the surface of the steel, and there is no attachments such as oxide scale, rust and paint coating that are not firmly attached. St3 very thorough hand and power rust removal. There should be no visible grease or dirt on the steel surface. And there are no attachments such as scale, rust and paint coating that are not firmly attached. Rust removal is more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster. (3) Flame rust removal, represented by the letter "F1", which includes using a power wire brush to remove the heated products attached to the steel surface after the flame heating operation. only one level: F1 Steel surfaces should be free of scale, rust and paint coatings, and any remaining traces should be merely surface discoloration (shades of different colors).

The steel surface rust removal grade is represented by letters representing the rust removal method used. A, B, C, and D in the front represent the original surface rust grade of the steel, and Arabic numerals in the back represent the clear degree grade, such as ASa2.5. Sa: Spray or blast rust removalSt: Hand and power tool rust removal FI: Flame rust removal 1. Spray or projectile rust removal 1. The most thorough cleaning level - Sa3. The cleaned steel surface appears completely consistent silver gray, with a certain surface roughness to improve surface adhesion. Grease, dirt, scale, rust, corrosion products, oxides and other impurities are completely removed from the surface. 2. Very thorough cleaning level - Sa2.5 There is no grease, dirt, scale, rust, corrosion products, oxides and other impurities on the cleaned surface. Shading or color difference on the surface due to incomplete cleaning is allowed, but at least 95% of the surface per square inch must reach the Sa3 cleaning level, and the remaining parts only have slight shading and color difference. 3. More thorough cleaning level - Sa2 There is no grease, dirt, rust scale and other impurities on the cleaned surface, and rust, scale and old paint are removed. Slight shading or color differences due to insufficient cleaning of rust and scale are allowed, up to a maximum area of ​​33% per square inch. If pitting corrosion has occurred on the surface of the steel, a small amount of rust or old paint is allowed to exist deep in the pits. 4. Non-thorough cleaning level - Sa1 The steel surface has been completely cleaned. Grease, dirt, loose scale and loose paint have been removed. Scale, rust, paint and coatings that are firmly bonded to the base material and cannot be removed with a very sharp blade are allowed to remain on the surface after cleaning. A large number of evenly distributed metal spots appear on the surface. 2. Rust removal by hand and power tools 1. St3 Very thorough rust removal by hand and power tools. The steel surface should be free of visible grease and dirt, and there should be no attachments such as scale, rust and paint coating that are not firmly attached. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster. 2. St2 Thorough manual and power tool rust removal The steel surface should be free of visible grease and dirt, and there should be no attachments such as scale, rust, and paint coatings that are not firmly attached. 3. Flame rust removal FI flame rust removal steel surface should be free of scale, rust, paint coating and other attachments, and any remaining traces should only be surface discoloration.

Classification of steel surface corrosion grades in my country* * Standard GB 8923-88 "Steel surface corrosion grade and rust removal grade before coating", and the Ministry of Chemical Industry, "Industrial Equipment and Pipeline Anti-Corrosion Engineering Construction and Acceptance Specifications" HGJ229-91 issued in July 1992 pointed out that the original rust degree of the uncoated steel surface is divided into four "rust grades" represented by A, B, C and D respectively. A. The steel surface is completely covered with oxide scale and has almost no rust. B. Steel surface that has been corroded and part of the oxide scale has peeled off. C. The oxide scale has peeled off due to rust, or can be scraped off, and there is a small amount of pitting on the steel surface. D. The oxide scale has been completely peeled off due to rust, and the surface of the steel has generally been pitted. Standard classification of steel surface rust removal (1) Surface treatment requirements for various types of anti-corrosion construction. The requirements for surface treatment quality level should comply with the following regulations. 1. For manual or power tool rust removal, the quality level of metal surface treatment is divided into two levels, represented by Sa1, Sa2 and Sa3. 2． For spray or projectile rust removal, the metal surface treatment quality level is set as level four, represented by Sa1, Sa2, Sa2.5, and Sa3. The rust removal quality level standards are shown in Table 3-8. 3． The quality level of flame rust removal metal surface treatment is set as level one, represented by F1. 4． The quality level of chemical rust removal metal surface treatment is set as level one, represented by Pi. : Rust removal quality grade standard Quality grade quality standard Sa3 The surface of the steel should be free of visible grease, dirt, scale, rust and paint coatings and other attachments. The surface should show a uniform metallic color, which is called spray or projectile rust removal to make the steel appear clean. Sa2.5 The surface of the steel should be free of visible grease, dirt, oxide scale, rust and paint coatings and other attachments. Any remaining traces should be only dots or stripes of slight stains, which is called very thorough spray or projectile rust removal. Sa2 The steel surface should be free of visible grease and dirt, and attachments such as oxide scale, rust and paint coatings have been basically removed, and the residues should be firmly attached. It is called thorough spray or projection rust removal Sa1. The steel surface should be free of visible grease and dirt, and there should be no attachments such as scale, rust and paint coatings that are not firmly attached. This is called light blasting or blasting.

3. Rust removal grade 3.1 General 3.1.1 The rust removal grade on the steel surface is represented by the letters “Sa”, “St” or “F1” representing the rust removal method used. If there are Arabic numerals after the letters, they indicate the degree of removal of scale, rust, paint coatings and other attachments. 3.1.2 For text descriptions of steel surface rust removal grades, see 3.2, 3.3 and 3.4. See Chapter 5 for photos of typical samples of these grades. Note: (1) In the definition of each rust removal grade in this chapter, the term "attachments" can include welding slag, welding spatter, soluble salts, etc. (2) In this chapter, when oxide scale, rust or paint coating can be peeled off from the steel surface with a metal putty scraper, it shall be regarded as weak adhesion. 3.2 Spray or projectile rust removal. 3.2.1 Spray or projectile rust removal is represented by the letter “Sa”. 3.2.2 Before spraying or projectile rust removal, thick rust layers should be removed. Visible grease and dirt should also be removed. After blasting or blasting rust removal, the surface of the steel should be cleared of floating dust and debris. 3.2.3 For steel surfaces that have been rust-removed by spraying or blasting, this standard stipulates four rust removal levels. The text description is as follows: Sal Mild spray or projection rust removal The steel surface should be free of visible grease and dirt, and there should be no attachments such as scale, rust, and paint coatings that are not firmly attached. See photos BSal, CSal and DSal. Sa2 Thorough spraying or blasting of rust-removed steel surfaces will result in no visible grease and dirt, and attachments such as scale, rust, and paint coatings have been basically removed, and their residues should be firmly attached. See photos BSa2, CSa2, DSa2. Sa2 * 0.5 Very thorough spraying or blasting rust removal will leave the steel surface free of visible grease, dirt, scale, rust, paint coatings and other attachments. Any remaining traces should be only dots or stripes of slight stains. See photo ASa2 * 0.5, BSa2 * 0.5, CSa2 * 0.5, and DSa2 * 0.5 Sa3 The steel surface should be free of visible grease, dirt, scale, rust, paint coating and other attachments to make the steel appear clean. The surface should show a uniform metallic color. See Photos ASa3, BSa3, CSa3 and DSa3 3.3 Hand and Power Tool Rust Removal 3.3.1 Rust removal using hand and power tools, such as scrapers, hand or power wire brushes, power sandpaper discs or grinding wheels, is represented by the letter "St". 3.3.2 Before derusting hand and power tools, thick rust layers should be removed, and visible grease and dirt should also be removed. After hand and power tool rust removal, the steel surface should be cleaned of loose dust and debris. 3.3.3 For steel surfaces that have been rusted by hand and power tools, this standard stipulates two rust removal levels, and the text description is as follows: St2 Thorough manual and power tool rust removal of steel surfaces should be free of visible grease and dirt, and free of loose scale, rust, paint coatings and other attachments. See photos BSt2, CSt2, and DSt2. St3 Very thorough hand and power tool rust removal. Steel surfaces should be free of visible grease and dirt, and free of loose scale, rust and paint coatings. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster. See photos BSt3, CSt3 and DSt3. 3.4 Flame rust removal 3.4.1 Flame rust removal is represented by the letters "FI". 3.4.2 Before flame rust removal, thick rust layers should be removed. Flame rust removal should include using power wire brushing to remove the products attached to the surface of the steel after the flame heating operation. 3.4.3 The textual description of the rust removal grade after flame rust removal is as follows: FI The surface of flame rust removed steel should be free of scale, rust, paint coating and other attachments, and any remaining traces should only be surface discoloration (shadows of different colors). See photos AFI, BFI, CFI, and DFI.