江阴达克罗金属制品有限公司

JiangYin Dacromet Metal Products Co., Ltd.

Assessment of surface cleanliness of steel

2020/7/8 10:07:58


江阴达克罗金属制品有限公司

JiangYin Dacromet Metal Products Co.,Ltd.


        Evaluation of steel surface cleanliness


In order to give full play to the protective and decorative role of coatings, it has been recognized that thorough surface treatment is necessary. The success of coating mainly depends on the quality of surface treatment. Generally, the higher the surface cleanliness (surface treatment quality) is, the better the protection effect of coating can be guaranteed, but too high requirements will also cause great waste. It is very important to evaluate the cleanliness of steel surface. The quality of surface treatment includes three aspects: visual cleanliness (rust, oxide scale, etc.), roughness and invisible cleanliness (grease, soluble iron salt, chloride, sulfide, dust, etc.) of steel plate surface. In this regard, the shipbuilding industry as the representative, has formed a relatively perfect detection standard and system, and other industries generally refer to the implementation.


Evaluation of visual cleanliness (rust and oxide scale) on steel surface


The first method is copper sulfate method: the copper sulfate solution is brushed on the surface of the treated steel plate, and the part with rust removed completely shows the color of metal copper, while the part with residual oxide skin larger than 0.5mm is dark, so as to judge the cleanliness of the surface. Copper sulfate solution can be prepared by adding 4-8 g cu2so4 to 1 g H2SO4 solution, or dissolving 36 g cu2so4 · 5H20 in 100ml water by heating, adding excess Cu (OH) 2 and placing it for 24 hours to remove excess Cu (OH) 2.


The second quantitative detection method is to use the different characteristics of oxide scale and iron resistance to measure the transition resistance between the treated surface and the probe (spherical pen electrode with a diameter of 1 mm), and judge the surface cleanliness by the average value of each point. In addition, a light reflection meter with a blue filter can also be used for surface cleanliness inspection. The quantitative measurement method of instrument is greatly affected by light, treatment method, original state and surface roughness, while copper sulfate method needs post-treatment, otherwise it will leave corrosion hazards. Therefore, the more reliable method is qualitative comparison with standard photos.


In order to correctly and conveniently evaluate the surface treatment quality of steel after derusting, many industrial developed countries have formulated the quality grade standards for steel derusting, among which the most significant is the Swedish industrial standard sis055900 "standard for rust removal of steel surface before coating", which has long been quoted by all countries in the world. ISO / TC, Technical Committee on paints and varnishes, ISO / TC Based on the Swedish standard sis055900-1967, the international standard iso8501-1:1988 "preparation of steel substrates before application of paints and related products - Visual assessment of surface cleanliness - Part 1: rust grades and derusting grades of uncoated steel and steel after full removal of the original coating" was formulated. The national standard is gb8923-88 "rust grades and derusting grades of steel surfaces before painting".


According to the above standards, the original corrosion degree of uncoated steel surface is divided into four "rust grades", and the quality of steel surface after rust removal is divided into several "rust removal grades". The rust grade and derusting grade of the steel surface are determined by the text description and the photos of the typical sample plate.

Corrosion grade


The original rust state of steel surface before derusting has a great influence on the difficulty of derusting and the surface appearance quality after derusting. Therefore, according to the oxide scale coverage and corrosion status of steel surface, the original corrosion degree is divided into four grades, represented by a, B, C and D, respectively.


A steel surface completely covered with oxide scale and almost free of rust.


B. steel surface that has been corroded and part of the oxide scale has peeled off.


C. steel surface with oxide scale peeled off due to corrosion or can be scraped off with a small amount of pitting corrosion.


D. the surface of steel which has been completely peeled off due to corrosion and pitting corrosion has occurred generally.


Derusting grade


The standard specifies the corresponding derusting grades for the steel surface cleanliness after spray or spray derusting, manual and power tool derusting and flame derusting, which are respectively represented by the letters SA, st and F1, and the Arabic numerals after the letters indicate the degree grades of removing oxide scale, rust, coating and other attachments.


The surface of SA1 mild spraying or throwing derusting steel shall be free of visible grease and dirt, and there shall be no attachment such as oxide scale, rust and coating which are not firmly attached.


The surface of steel should be free of visible grease and dirt after SA2 thorough spraying or spraying, and the attachments such as oxide scale, rust and paint coating have been basically removed, and the residues should be firmly attached.


Sa2.5 the steel surface shall be free of visible grease, dirt, oxide scale, rust, paint coating and other attachments, and any residual trace shall be only spot or stripe light spot.


The surface of the steel should be free of visible grease, dirt, oxide scale, rust, paint coating and other attachments, and the surface should show uniform metal color.


ST2 thoroughly remove rust by hand and power tools. The steel surface shall be free of visible grease and dirt, and there shall be no attachment such as oxide scale, rust and paint coating which are not firmly attached.


ST3 is a very thorough manual and power tool derusting method. The steel surface should be free of visible grease and dirt, and there should be no attachment such as oxide scale, rust and paint coating which are not firmly attached. The rust removal should be more thorough than that of ST2. The surface of exposed part of substrate should have metallic luster.


F1 flame derusting steel surface should be free of oxide scale, rust, paint coating and other attachments. Any residual trace is only surface discoloration (shadow of different colors)


In the standard, there are 4 typical samples showing the original corrosion degree of steel plate surface, and 24 photos showing the rust removal grade achieved by spraying or throwing rust removal, manual and power tool derusting and flame derusting. These photos are marked with the symbols of the original rust grade before rust removal and the rust removal grade after rust removal. They are as follows:


A B C D


BSt2 BSt3 CSt2 CSt3 DSt2 DSt3 ASa2.5 ASa3BSa1 BSa2 BSa2.5 BSa3 CSa1 CSa2 CSa2.5 CSa3 DSa1 DSa2 DSa2.5 DSa3


For example, if the original corrosion grade of the steel is grade B and the rust is removed to Sa2.5 by sand blasting, the corresponding photo is marked as bsa2.5.



The standard does not contain photos of grade asa1, ASA2, ast2 and ast3, because the minimum requirements for painting can not be met by derusting to these grades on the steel surface with original corrosion state of Grade A.


The above standards are based on the visual appearance of the steel surface to express the rust grade and rust removal grade. Therefore, the evaluation of these grades should be carried out under good scattered sunlight or artificial lighting conditions with equivalent illumination. The examiners should have normal vision. At the same time, it is stipulated in the standard that magnifying glass and other instruments should not be used for grading.


When evaluating the rust grade and derusting grade, the steel surface to be inspected shall be compared with the corresponding photos visually, and the photos shall be as close as possible to the surface to be inspected. If the original corrosion grade of the steel is evaluated, the rust grade indicated in the corresponding grade photo with relatively serious corrosion shall be taken as the evaluation result; if the rust removal grade is evaluated, the rust removal grade indicated in the photo closest to the steel surface appearance shall be taken as the evaluation result. For example, if the actual corrosion degree of a steel plate is between photo B and photo C, the corrosion grade of the steel plate shall be determined as grade C. There are many factors that will affect the result of our visual evaluation by visual inspection. For example, the 14 photos of rust removal by spraying and throwing in the standard are the steel surface appearance after dry spray derusting with quartz sand abrasive. When other kinds of abrasives are used, such as steel shot, steel sand, steel wire segment and copper slag, the surface color is darker. In contrast, iso8501-3 standard provides a set of typical sample photos reflecting the difference of steel surface appearance after different abrasive jet treatment, as a supplement to iso8501-1 standard. The standard provides six kinds of commonly used Abrasives: typical sample photos of the appearance of low carbon steel with corrosion grade C treated to csa3. The six abrasives are: ① high carbon steel shot, grade Ig 100, Vickers hardness 400-520hv; ② iron sand, grade Ig 070, Vickers hardness 400-520hv; ③ iron sand, grade Ig 070, Vickers hardness 750-950hv; ④ chilled cast iron sand, grade Ig 070; ⑤ copper slag; and (6) coal furnace slag.