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不锈钢酸洗作业指导书 AvestaPolarit Welding Pickling handbook Handbook for the pickling and cleaning of stainless steel Index FOREWORD 1 1 STAINLESS STEEL AND THE NEED FOR CLEANING 2 1.1 Introduction 2 1.2 Typical defects 2 1.2.1 Heat tint and oxide scale 2 1.2.2 Weld defec...

不锈钢酸洗作业指导书
AvestaPolarit Welding Pickling handbook Handbook for the pickling and cleaning of stainless steel Index FOREWORD 1 1 STAINLESS STEEL AND THE NEED FOR CLEANING 2 1.1 Introduction 2 1.2 Typical defects 2 1.2.1 Heat tint and oxide scale 2 1.2.2 Weld defects 2 1.2.3 Iron contamination 2 1.2.4 Rough surface 2 1.2.5 Organic contamination 2 2 CLEANING PROCEDURES 3 2.1 Mechanical methods 3 2.1.1 Grinding 3 2.1.2 Blasting 3 2.1.3 Brushing 3 2.1.4 Summary 3 2.2 Chemical methods 4 2.2.1 Electropolishing 4 2.2.2 Pickling 4 2.2.3 Summary 5 2.2.4 Passivation and decontamination 5 2.3 Choice of method 5 3 CHEMICAL METHODS IN PRACTICE 6 3.1 General requirements 6 3.2 Pre-cleaning/degreasing 6 3.3 Pickling 6 3.3.1 Pickling with paste/gel 7 3.3.2 Pickling with solution 8 3.3.3 Pickling in a bath 9 3.4 Passivation and decontamination 10 4 NEUTRALISATION AND WASTE TREATMENT 10 4.1 Neutralisation 10 4.2 Waste treatment 10 5 INSPECTION AND TROUBLESHOOTING 11 5.1 Test methods 11 5.2 Troubleshooting 11 6 SAFE HANDLING AND STORAGE OF PICKLING PRODUCTS 12 6.1 Safety rules 12 6.2 Personal safety 12 6.3 Storage 13 REFERENCES 14 DISCLAIMER Inside cover Author: Anders Bornmyr, AvestaPolarit Welding AB Co-author: Björn Holmberg, AvestaPolarit AB © AvestaPolarit Welding AB. First edition 1995. Copyright: All rights reserved. No part of this handbook may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, or translated into any other language without the prior written permission of AvestaPolarit Welding AB. 1 Foreword AvestaPolarit is one of the world´s major manufacturers of stainless steel. The parent company´s production of plate, sheet, coils and billets provides the basis for processing operations by subsidiaries and associated companies producing piping, fittings and manufactured goods as well as rod, wire and welding consumables. The AvestaPolarit Group includes AvestaPolarit Welding , one of Europe’s largest producers of welding consumables and pickling products for stainless steel and special alloys. In this handbook, AvestaPolarit Welding presents practical methods for pickling and cleaning stainless steel. Appropriate safety procedures when handling the products involved are also described. The aims of this handbook are to provide a generally increased awareness and understanding of the need for surface treatment of stainless steel and in particular: - to explain why stainless steel structures need cleaning after welding and processing in order to preserve their corrosion resistance, - to show when cleaning is important through a survey of typical defects, - to describe how to clean using different cleaning techniques, and - to give practical recommendations and instructions as to what to do in order to eliminate typical drawbacks. 2 1 Stainless steel and the need for cleaning 1.1 INTRODUCTION A stainless steel surface should appear clean, smooth and faultless. This is obvious when the steel is used for such purposes as façades or in applications with stringent hygienic requirements, but a fine surface finish is also crucial to corrosion resistance. Stainless steel is protected from corrosion by a thin, impervious, invisible surface layer – the passive layer – that consists mainly of chromium oxide. The oxygen content of the atmosphere or aerated aqueous solutions is normally sufficient to create and maintain this passive layer. Unfortunately, surface defects and imperfections introduced during manufacturing operations may drastically disturb this ”self-healing” process and reduce resistance to several types of local corrosion. This means that a final cleaning process will often be required to restore an acceptable surface quality with regard to hygiene and corrosion. The extent of and methods for post-manufacture treatment will be determined by the corrosivity of the environment, the corrosion resistance of the steel grade, hygienic requirements (e.g. in the pharmaceutical and food industries) or by purely aesthetic considerations. Consideration must also be paid to local environmental requirements. Both chemical and mechanical cleaning methods are available. Good design, planning and methods of manufacture can reduce the need for finishing work and thus reduce costs. The influence of defects, and ultimately their removal, must be considered when manufacturing to specifications that relate to certain surface quality requirements. For further details and explanations, the ”Avesta Sheffield Corrosion Handbook” is recommended. 1.2 TYPICAL DEFECTS 1.2.1 Heat tint and oxide scale High temperature oxidation – caused by processes such as heat treatment or welding – produces an oxide layer with inferior protective properties, compared with those of the original passive layer. A corresponding chromium depletion in the metal immediately below the oxide also occurs. The chromium-depleted zone under normal welding heat tint is very thin and can normally be removed together with the tint. It is, however, necessary to remove this layer in order to completely restore corrosion resistance. 1.2.2 Weld defects Incomplete penetration, undercut, pores, slag inclusions, weld spatter and arc strikes are typical examples of weld defects. These defects have negative effects on mechanical properties, resistance to local corrosion and make it difficult to maintain a clean surface. The defects must therefore be removed, normally by grinding, although sometimes repair welding is also necessary. 1.2.3 Iron contamination Iron particles can originate from machining, cold forming and cutting tools, blasting grits/sand or grinding discs contaminated with lower alloyed material, transport or handling in mixed manufacture, or simply from iron-containing dust. These particles corrode in humid air and damage the passive layer. Larger particles may also cause crevices. Reduced corrosion resistance will result in both cases. This type of corrosion produces unsightly discoloration and may also contaminate media used in the equipment in question. Iron contamination can be detected using the ferroxyl test; see chapter 5. 1.2.4 Rough surface Uneven weld beads and grinding or blasting too heavily will result in rough surfaces. A rough surface collects deposits more easily, thereby increasing the risk of both corrosion and product contamination. Heavy grinding also introduces high tensile stresses, which increase the risk of stress corrosion cracking and pitting corrosion. There is a maximum allowed surface roughness (Ra-value) for many applications, and manufacturing methods that result in rough surfaces should generally be avoided. 1.2.5 Organic contamination Organic contaminants in the form of grease, oil, paint, footprints, glue residues and dirt can cause crevice corrosion in aggressive environments, render surface pickling activities ineffective, and pollute products handled in the equipment. Organic contaminants should be removed using a suitable pre-cleaning/ degreasing agent (chlorine-free). In simple cases, a high-pressure water jet can be used. 3 2.1 MECHANICAL METHODS 2.1.1 Grinding Grinding is normally the only method that can be used to remove defects and deep scratches. A grinding disc is usually adequate for treating defects of this type. The grinding methods used should never be rougher than necessary, and a flapper wheel is often sufficient for removing weld tint or surface contamination. The following points must always be considered: • Use the correct grinding tools – self-sharpening, iron-free discs should always be used for stainless steel – and never use discs that have previously been used for grinding low alloy steels. • Avoid producing a surface that is too rough. Rough grinding with a 40-60 grit disc should always be followed by fine grinding using, for example, a higher grip mop or belt to obtain a surface finish corresponding to grit 180 or better. If surface requirements are very exacting, polishing may be necessary. • Do not overheat the surface. Apply less pressure when grinding in order to avoid creating further heat tint. • Always check that the entire defect has been removed. 2 Cleaning procedures Different chemical and mechanical methods, and sometimes a combination of both, can be used to remove the defects mentioned. Generally, cleaning based on chemical methods can be expected to produce superior results since most effective mechanical methods tend to produce a rougher surface whilst chemical cleaning methods reduce the risk of surface contamination. Local regulations in respect of environmental and industrial safety as well as waste disposal problems may, however, limit their application. 2.1.2 Blasting Sand and grit blasting (peening) can be used to remove high temperature oxide as well as iron contamination. However, care must be taken to ensure that the sand (preferably of olivine type) or grit is perfectly clean. The blasting material must therefore not have been previously used for carbon steel; not should the sand or grit be too old, since it becomes increasingly polluted, even if it has only been used for blasting contaminated stainless steel surfaces. The surface roughness is the limiting factor for these methods. Using low pressure and a small angle of approach, a satisfactory result can be achieved for most applications. For the removal of heat tint, shot peening using smooth glass beads produces a good surface finish and introduces compressive stresses which improve stress corrosion cracking resistance and resistance to fatigue. 2.1.3 Brushing For the removal of heat tint, brushing using stainless steel or nylon brushes usually provides a satisfactory result. These methods do not cause any serious roughening of the surface, but do not guarantee complete removal of the chromium-depleted zone. As regards the other mechanical methods, the risk of contamination is high, and it is therefore important that clean tools that have not been used for processing carbon steels are used. 2.1.4 Summary A final mechanical cleaning stage following a typical manufacturing programme could be as follows: • Removal of welding defects by grinding; • Removal of material affected by high temperatures and, if possible, removal of iron impurities. The surface must not become unacceptably rough; • Removal of organic contaminants (see section 1.2.5); • A final acid treatment – passivation/ decontamination – is strongly recommended. A thorough rinsing with fresh water, preferably using a high-pressure water jet must follow the acid treatment. In exceptional cases, however, rinsing by high-pressure water jet only may suffice as the final treatment. Figure 1. Surface defects Weld metal Parent material Residual slag Undercut Tarnish Iron contamination Spatter Organic contaminants 4 Group International AvestaPolarit AvestaPolarit DIN SS Pickleability steel number/name steel name chemical composition, average % EN ASTM C Cr Ni Mo Others 1 1.4301 304 4301 0.04 18.1 8.3 – – 1.4301 2333 1 1.4401 316 4401 0.02 17.2 10.2 2.1 – 1.4401 2347 2 1.4404 316L 4404 0.02 17.2 10.2 2.1 – 1.4404 2348 2 1.4571 316Ti 4571 0.04 16.8 10.9 2.1 Ti 1.4571 2350 2 1.4436 316 4436 0.02 16.9 10.7 2.6 – 1.4436 2343 2 2 1.4362 S32304 SAF 2304™ 0.02 23 4.8 0.3 – 1.4362 2327 3 1.4462 S32205 2205 0.02 22 5.7 3.1 – 1.4462 2377 3 1.4439 S31726 4439 0.02 17.8 12.7 4.1 – 1.4439 – 3 1.4539 N08904 904L 0.01 20 25 4.3 1.5 Cu 1.4539 2562 3 3 1.4410 S32750 SAF 2507™ 0.02 25 7 4 – – 2328 4 1.4547 S31254 254 SMO® 0.01 20 18 6.1 Cu – 2378 4 1.4652 S32654 654 SMO® 0.01 24 22 7.3 3.5 Mn, Cu – – 4 2.2 CHEMICAL METHODS Chemical methods can remove high temperature oxide and iron contamination without damaging the surface finish. Electropolishing may improve the surface finish. Since they remove the surface layer by controlled corrosion, chemicals will also selectively remove the least corrosion-resistant areas such as the chromium-depleted zones. After the removal of organic contaminants (section 1.2.5), the following procedures are commonly used. 2.2.1 Electropolishing Electropolishing normally produces a surface that guarantees optimal corrosion resistance. The material gains a fine lustre, and, above all, an even micro- profile that meets extremely stringent hygienic requirements. 2.2.2 Pickling Pickling is the most common chemical procedure used to remove oxides and iron contamination. Thorough rinsing with clean tap water must follow pickling. The water quality requirements, including acceptable chloride content, increase with the surface requirements. Pickling normally involves using an acid mixture containing 8-20 vol% nitric acid (HNO3) and 0.5-5 vol% hydrofluoric acid (HF). Chloride- containing agents such as hydrochloric acid (HCl) should be avoided, since there is an obvious risk of pitting corrosion. The effectiveness of pickling depends on the following factors: • The surface. This must be free of organic contamination; • The temperature. The effectiveness of the acids increases strongly with temperature. This means, for example, that the pickling rate can be increased considerably by increasing the temperature. There are, however, upper temperature limits that must also be considered. See below. • The composition and concentration of the acid mixture. • The steel grade. Highly alloyed grades need a more aggressive acid mixture and/or higher temperature in order to avoid an excessively long pickling time. See table 1. • The thickness and type of the oxide layer. This depends largely on the welding procedure used. Welding using an effective shielding gas will produce a minimum of weld oxides. Such a gas should be as free of oxygen as possible. For further information, see the AvestaPolarit Welding Handbook on welding stainless steel. Mechanical pre-treatment to break or remove the oxide might be advisable, particularly when pickling highly alloyed steel grades. • The surface finish. A rough hot rolled surface may be harder to pickle than a smooth cold rolled one. A number of different pickling methods can be used: • Pickling in a bath is a convenient method if suitable equipment is available. The composition of the acid mixture and the bath temperature (20-65ºC) are chosen with regard to the stainless steel grade and the type of heat oxide. Overpickling, resulting in a rough surface, may result when pickling the lowest alloyed stainless grades at excessive temperatures. The steels have been divided into three groups. The pickleability of these steel grades ranges from 1 (very easy) to 4 (very difficult). SAF 2304 and SAF 2507 are manufactured under licence granted by AB Sandvik Steel. Table 1. Stainless steel grades and their pickleability 5 The effectiveness of pickling is influenced not only by the acid concentration and the temperature, but also by the free metal content (mainly iron) in the bath. An increased iron content requires a higher bath temperature. A rough guideline is that the free iron (Fe) content measured in g/l should not exceed the bath temperature (ºC). When metal contents in the bath reach excessive levels (40-50 g/l), the bath solution can be partially or totally emptied out and fresh acid added. • Pickling with pickling paste. Pickling paste for stainless steels consists of an acid mixture (normally HF/HNO3) with added binding agents. It is suitable for pickling limited areas, e.g. weld-affected zones. It is normally applied using an acid-resistant brush. The paste is not effective at low temperatures (5-10ºC). The risk of overpickling at high temperatures is less than when using bath pickling. A greater risk is that of the paste drying out due to evaporation, resulting in reduced pickling effect and rinsing difficulties. Objects should therefore not be pickled at temperatures higher than 40ºC or in direct sunlight. Rinsing with water should be carried out before the paste dries. Even if neutralisation of the pickling paste is carried out on the metal surface for environmental and practical reasons, a thorough rinsing with water is vital. • Pickling with pickling solution. Pickling solution (or pickling gel in spray form) normally consists of a mixture of nitric acid and hydrofluoric acids (phosphoric acid can be used to obtain mild pickling properties), with binding agents and surface-active agents to obtain good thixotropy and the right viscosity. It is suitable for pickling large surfaces, e.g. when the removal of iron contamination is also desired. 2.2.3 Summary A final pickling/cleaning operation following a typical manufacturing programme could be: • Grinding for removal of defects caused by welding. It is important that slag is removed after welding. • Removal of organic contamination (section 1.2.5). • Pickling using a bath, paste or solution, possibly in combination with a careful mechanical treatment to break oxides. • A thorough rinsing with water, preferably using a high-pressure water jet. 2.2.4 Passivation and decontamination This procedure is carried out in a manner similar to pickling, but in this case the active agent is nitric acid only, 18-30 weight % at around 20ºC. The acid is applied by immersion or spraying. This treatment strengthens the passive layer. The treatment is more important after mechanical cleaning and operations involving a risk of iron contamination, since the acid also removes iron impurities from the surface. Consequently, the method could also be referred to as decontamination. As after every acid treatment, rinsing with water is vital. 2.3 CHOICE OF METHOD The choice of method and the extent of final cleaning required will depend on the need for corrosion resistance, hygienic considerations (pharmaceuticals, food) or whether visual appearance is the sole criterion. The routine removal of welding defects, welding oxides, organic substances and iron contaminants is normally a basic requirement and usually allows a comparatively free choice of final treatment. Provided that the surface roughness so permits, both mechanical and chemical methods can be used. However, if an entirely mechanical cleaning method is considered, the manufacturing stage has to be very well planned in order to avoid iron contamination, since decontamination, probably with nitric acid, will otherwise be necessary (section 2.2.4). When requirements as to surface finish and corrosion resistance are exacting, the choice of method is more critical. A treatment sequence based on pickling (section 2.2.3) will in such cases provide the best chances of a superior result. Figure 2. Pickling offers better results than alternative surface treatments such as grinding and polishing. Grinding Polishing Pickling 6 3 Chemical methods in practice 3.1 GENERAL REQUIREMENTS The practical use of cleaning chemicals is demanding, and certain working procedures need to be followed. The choice of chemical cleaning process mainly depends on the type of contaminants and heat oxides to be removed, the degree of cleanness required and the cost. This chapter gives guidelines for the application of suitable chemical cleaning procedures. In order to avoid health hazards or environmental problems, pickling should be carried out in a special pickling area indoors. In this context, the following recommendations should be met: • Handling instructions, essential product information, such as product labels, and safety data sheets for the various products must be available. Local and national regulations should also be available. See also section 6.1. • Responsible staff should be familiar with the health hazards associated with the products and how these should be handled. • Personal safety equipment, including suitable protective clothing and facemask should be used. See also section 6.2. • When pickling indoors, the workplace should be separated from other workshop operations in orde
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