Post-weld cleaning of duplex stainless steel - a review of the process

POST-WELD CLEANING OF DUPLEX STAINLESS STEEL - A REVIEW OF THE PROCESS The importance of post welding cleaning and its influence on the corrosion resistance of welded DSS Opracował: dr hab. inż. Zbigniew Brytan Katedra Materiałów Inżynierskich i Biomedycznych, Wydział Mechaniczny Technologiczny, Politechnika Śląska, Gliwice SYMPOZJUM KATEDR I ZAKŁADÓW SPAWALNICTWA pt. Nowoczesne zastosowania technologii spawalniczych Post-weld cleaning of duplex stainless steel - a review of the process Opracował: dr hab. inż. Zbigniew Brytan 1 Abstract The duplex stainless steels as all corrosion resistant alloys require careful surface treatment after the completion of the welding stage. Incorrect performance of this stage of material processing may drastically reduce corrosion resistance and lead to numerous local corrosion phenomena, despite the correct welding technology itself. Therefore, knowledge of proper post-welding surface cleaning is essential to achieve the expected performance of duplex stainless steel welded parts. The review of main processes and methods applied during post-welding cleaning of duplex stainless steel surface is described in the paper. The mechanical and chemical methods are discussed. The importance of such process and its influence on the corrosion resistance is discussed. The influence of surface finishing method on the stainless steel surface corrosion resistance is presented. Keyworks: duplex stainless steel, post welding surface treatment, heat ting The importance of post-weld cleaning of duplex stainless steel Stainless steel is protected from external environment by a thin passive layer, that consists mainly of chromium oxide and hydroxide. The oxygen content present in the atmosphere or aqueous solutions is normally sufficient to create and maintain this protective layer. Unfortunately, surface defects and imperfections introduced during welding may drastically reduce resistance to local corrosion phenomena. This means that a final cleaning process is required to restore an acceptable surface quality with regard to hygiene and corrosion requirements. The typical weld surface defects, that should be restored during post-welding treatment, include heat tint and oxide scale on the surface, weld defects, iron contaminations, organic contaminations, and a rough surface in welded zone. These defects can initiate corrosion on the duplex plate surface and must be removed. That is why post-fabrication clean-up is very important and shall be the same as for other families of stainless steels [1]. Mechanical surface treatment methods such as fine abrasive grinding are commonly used to preliminary remove any contamination of the welded areas caused by the welding operation. Ferrous contamination can be removed by chemical cleaning with nitric acid. That is why the carbon steel tooling must be avoided as much as possible when working with stainless steel to decrease the risk of iron contamination. Organic contamination (oil, grass, paint, etc.) can be removed with a chlorine-free solvent [1,2]. With exposure to oxygen at high temperature, a thin “straw tinted” chromium oxide layer form. One of the most crucial surface effects, regarding corrosion performance of welded DSS, is related to weld high temperature oxidation and formation of an oxide layer with inferior protective properties, compared with those of the original passive layer (fig. 1). As a result, a chromiumdepleted layer forms beneath the heat tint and may have a reduced corrosion resistance. The metal surface directly under the oxide layer (thickness 0.02 ÷ 0.2 µm) shows a depletion of chromium and is known as chromium-depleted zone. When cleaning the surface after welding, both heat tint discoloration and the chrome-depleted zone underneath should be removed. It is necessary to remove this layer to completely restore the DSS corrosion resistance in the weld heat affected zone. Heat tint oxides can be removed by pickling using commercial picking pastes or by means of immersion in pickling HNO3/HF solution bath [2,3]. The weld defects (i.e., pores, slag inclusions, weld spatter, etc.) have negative effects on resistance to local corrosion and mechanical properties. Therefore, they must be removed, normally by mechanical surface treatment, typically grinding. Similarly, iron contamination must be removed from stainless steel surface. Iron particle in contact with noble stainless steel surface corrode in the SYMPOZJUM KATEDR I ZAKŁADÓW SPAWALNICTWA pt. Nowoczesne zastosowania technologii spawalniczych Post-weld cleaning of duplex stainless steel - a review of the process Opracował: dr hab. inż. Zbigniew Brytan 2 presence of electrolyte (humid air) and damage the passive layer, what result in reduced corrosion resistance. This type of corrosion at the beginning produces only surface discoloration, but with the passage of time and lack of cleaning, a favourable area is created for initiation of crevice and pitting corrosion on the surface. In turn, organic contamination of the surface in the form of greases, paints, dirt, etc. can cause crevice corrosion in aggressive media. For this reason, they should be removed from the surface, especially for effective chemical treatment (surface pickling), as they can render pickling process ineffective. Fig. 1. Oxide layer formation on the welded surface [4] Cleaning of stainless steel weldments is important to achieve adequate corrosion resistance after welding with particular emphasis being given to the heat affected-zones. Methods such as mechanical and chemical cleaning are typically used. Electrochemical methods are also used where low surface roughness of cleaned area is highly required. After welding the surface condition of the metal should be restored by methods that include brushing, grinding, pickling, blasting. Combinations of these processes can be used to produce surfaces with the required corrosion resistance. The chemical surface treatment of welded duplex stainless steel requires various methods depending on surface contamination level. The uneven weld beads and further mechanical surface finishing of the welded zone can result in rough surfaces. Rougher surface, more deposits is collected at that thereby increase the risk of corrosion initiation (fig. 2). When mechanical finishing also introduces high tensile stresses in metal surface, the risk of stress corrosion cracking and pitting corrosion is further increased. Manufacturing methods that result in rough surfaces should generally be avoided. For the above reasons there is a maximum allowed surface roughness (Ra value) for many applications, which will ensure consistently high surface corrosion resistance. The corrosion resistance of the stainless steel is affected by the surface roughness after grinding/polishing, with a marked decrease in the corrosion resistance as the surface roughness increases above a Ra value of about 0.5 micrometres. This roughly corresponds to the surface produced by grinding with 320 grit abrasives. The acceleration of the corrosion of the surface at Ra above 0.5 micrometres is apparent. Fig. 2. Influence of surface roughness on the corrosion resistance of stainless steels [5] SYMPOZJUM KATEDR I ZAKŁADÓW SPAWALNICTWA pt. Nowoczesne zastosowania technologii spawalniczych Post-weld cleaning of duplex stainless steel - a review of the process Opracował: dr hab. inż. Zbigniew Brytan 3 To remove heat tint and restore the corrosion resistance of duplex stainless steel surfaces, they are pickled after welding. The pickling process involves the following steps: cleaning and degreasing, pickling, passivating, rinsing. Pickling and passivation can be done using several methods, including immersion pickling, paste pickling, spray pickling. Mixtures of nitric and hydrofluoric acid are used for pickling. After pickling and passivation components must be thoroughly rinsed by water. This will ensure that all the pickling and passivation solution used has been fully removed. When brushing DSS surface a suitable stainless steel brushes must be used to heat tint removing. Once used for stainless steel surface treatment, the stainless steel brushes should be reserved for further stainless steel work only. Mild steel or brass brushes must not be used for brushing DSS surfaces. Surface rust staining can occur in storage or service if these guidelines are not followed. Grinding of weld seams to remove heat tinting should be done with wheels that are not too coarse, used at a low surface pressure. Residual stresses can be left in the metal if wheels that are too coarse or too high a surface pressure are used for grinding. In service these residual stresses can result in stress corrosion cracking failures. The grinding wheels used should be of the types specifically intended for grinding stainless steels. They should not contain any iron constituents. Shotblasting of DSS surfaces is normally done using glass beads with a diameter of approximately 0.3 mm. The blasting medium must only be used once. After shotblasting pickling is normally done to produce a clean, smooth, matt finished surface. Pickling of welds and embedded surface contamination is an important consideration to maintain optimum corrosion resistance performance for any fabricated duplex stainless steel material. It becomes even more important if the application is an aggressive pitting or crevice environment. Pickling with a HNO3/HF solution can remove heat tint and embedded carbon steel particles, both of which can affect the corrosion performance of the material. Duplex stainless steels seem to exhibit more sensitivity to heat tint than austenitic stainless steels, so pickling of the welds should be considered if the tint is straw brown or darker (fig. 3). It should also be noted that duplex materials are generally more resistant to pickling solution and, therefore, may require more pickling time to achieve the expected results. It is recommended to use welding gases with O2 content below 200 ppm for duplex stainless steels and to flush the pipes from the root side with forming gas in order to reduce the O2 content as much as possible. In practice recommendations suggest a reduction of O2 concentration below 50 ppm and even below 25 ppm in order to limit heat tint formation. Another technique used to remove the oxide layers formed on the surface by welding is electropolishing process. Electropolishing is an effective method of surface cleaning, which also results in removing oxides and iron from the metal surface. Beside that it also preferentially dissolves iron and hence improves the Cr-Fe ratio of the surface layers. It alters the surface appearance by smoothing. The electrochemical polishing of the surface significantly improves the corrosion resistance of welded stainless steels. Electropolishing results in a clean, fully passivated surface which is generally superior to that produced using conventional pickling or passivation solutions and especially by mechanical methods (brushing, grinding) (fig. 4). The importance of the surface roughness and chemical surface treatment on the corrosion resistance of stainless steel can be analysed based on the G. Coates [8] results, shown in figure 5. G. Coates [8] reported that surface finish is an important additional factor that influences the resistance to initiation of pitting and possible crevice corrosion of stainless steels. It has also been demonstrated that oxide removal by adequate mechanical and/or chemical cleaning can restore the corrosion properties close to those of the bulk material. Among the analysed methods the pickling process has proven to be the most effective one. As shows in the figure 5, the pickled or passivated SYMPOZJUM KATEDR I ZAKŁADÓW SPAWALNICTWA pt. Nowoczesne zastosowania technologii spawalniczych Post-weld cleaning of duplex stainless steel - a review of the process Opracował: dr hab. inż. Zbigniew Brytan 4 rough, matt 36 grit finish shows better corrosion resistance than an untreated 220 grit finish. The overall conclusion is that weld discoloration has to be removed to maintain the corrosion resistance and that mechanical cleaning followed by pickling gives the best result. Fig. 3. The heat tint recognition chart for stainless steel welding. Recommended surface weld decolourisation level according to AWS D18.2 Guidelines [6] Fig. 4. The influence of surface finishing method on the stainless steel surface corrosion resistance The final process and the most important for the corrosion resistance of the stainless steel surface is passivation. Passivation can occur naturally or accelerated with the use of oxidizing SYMPOZJUM KATEDR I ZAKŁADÓW SPAWALNICTWA pt. Nowoczesne zastosowania technologii spawalniczych Post-weld cleaning of duplex stainless steel - a review of the process Opracował: dr hab. inż. Zbigniew Brytan 5 chemicals, as is the case in industrial practice. Passivation is the process of forming a natural, protective, corrosion resistant layer on stainless steel surfaces. This is promoted by cleaning the surface of dirt, grease, oil, salts, and iron contaminants. Water, detergents, and mechanical cleaning techniques can readily remove dirt, oils, and salts on the metal surface. Iron and iron oxides are not so readily removed with standard cleaning methods, hence pickling and passivating solutions containing phosphoric, citric, nitric, and hydrofluoric acid are often used for this purpose. Fig. 5. The effect of chemical and mechanical surface treatments on corrosion resistance of stainless steel (316L austenitic stainless steel) [7,8] It should be noted that the weld is always more susceptible to corrosion than the parent material. That is due to many factors, including those related to the welding technology and the weld material's resulting structure, the fraction of austenite and ferrite, and the applied post welding surface treatment/cleaning treatment welded zone. The critical pitting temperature (CPT) of the weld surface for stainless steels is lower than that of the parent material (fig. 6). As the welding can decrease the corrosion resistance of welded DSS, special attention should be paid when performing the post welding surface finish treatment. Fig. 6. Critical pitting temperature (CPT) determined in the ASTM G48A test for the mase metal and weld material of austenitic and duplex stainless steels [9] SYMPOZJUM KATEDR I ZAKŁADÓW SPAWALNICTWA pt. Nowoczesne zastosowania technologii spawalniczych Post-weld cleaning of duplex stainless steel - a review of the process Opracował: dr hab. inż. Zbigniew Brytan 6 Passivation is the treatment of the surface of stainless steels, often with acid solutions (or gels/pastes), to remove contaminants and promote the formation of the passive film on a freshly created surface, e.g., through grinding, machining, or mechanical damage. The passivation process works by dissolving any carbon steel contaminations (iron particles) from the stainless steel surface and dissolving out sulphide inclusions present on the surface. Nitric acid (HNO3) may also enrich the chromium proportion at the surface (chromium to iron ration). Passivation treatments include HNO3 solutions or pastes, which will clean the steel surface free of iron contaminants. The passivating compositions contain up to about 30% HNO3 and may contain other oxidizers. The passivation process does not affect the appearance of the stainless steel surface. Nitric acid is generally used and involves several ecological problems, citric acid could be a promising and environmentally friendly alternative to nitric acid. The key parameters which can be used to identify the passivated stainless steel are the total Cr/Fe atomic ratio, the Cr oxide/Fe oxide ratio, and the total oxide thickness (fig. 7). The XPS studies [10] of passivated and un-passivated steel surface shows that passivation significantly changes the ratio of iron oxide and metallic states. The Cr/Fe ratio in oxide layer increase due to passivation. The un-passivated surface shows Cr/Fe ratio about 0.77, in contrast passivated surface Cr/Fe=1.56. Therefore, the Cr/Fe ratio and Cr/Fe oxide ratios are useful parameters used to assess the passivation process's quality. Concluding, the goal of the passivation process is then to maximize the chromium-containing layer on the surface. Fig. 7. The chemical composition depth profile of un-passivated and passivated steel surfaces [10] Stainless steel surfaces have been successfully passivated in various citric acid solutions (commercial, waste extracted citric acid) and demonstrated the potentiality of citric acid-based passivation [11÷13]. The citric acid passivation treatment significantly increases the corrosion resistance due to a high Cr content in the passive film. Citric acid passivation is almost effective as well as nitric acid passivation. The citric acid passivation treatments, described in passivation standard ASTM A967, covers various batch composition (20÷25% 38÷42% vol. of citric acid and addition of sodium dichromate) and batch temperatures ranging from 20 to 70°C. Besides, a composition with the addition of chelants was developed that also need to be employed at elevated temperatures (60÷80°C) to be effective. Citric acid with chelants produces an even better passive film with more chromium in the surface than the other treatment methods. Citric acid passivation produces passive film with a Cr/Fe ratio typically of 1.7 to 2.0 [19]. This ratio is the standard quality level of passive film that is specified in most industry standards, and it meets the requirements of ASTM A380 and A967. In contrast, nitric acid passivation produces a SYMPOZJUM KATEDR I ZAKŁADÓW SPAWALNICTWA pt. Nowoczesne zastosowania technologii spawalniczych Post-weld cleaning of duplex stainless steel - a review of the process Opracował: dr hab. inż. Zbigniew Brytan 7 passive film of approximately 1.4 to 1.6 Cr/Fe. While using phosphoric acid in electropolishing process will generate a passive film of 1.1÷1.3 Cr/Fe [12]. Passivation processes with citric acid are also of great interest to producers of chemicals for the chemical treatment of stainless steels, which indicates the potential of such a solution as an alternative to the use of nitric acid. Although known for many years, passivation of the steel surface with citric acid has not been widely used in industrial conditions so far. Due to environmental aspects, a significant increase in interest in this passivation method can be noticed in recent years. Passivation with citric acid has many advantages, including a limited emission of nitrogen oxides NOx, increasing personnel's safety, and meets the environmental protection requirements included in the latest environmental regulations. Another advantage is the more remarkable ability to effectively remove iron from the processed stainless steels' surfaces, which means that less concentrated acids are used. Conclusions Duplex stainless steels, especially in the welded conditions, resist corrosion best if they are clean and smooth. Clean stainless steel means free of contaminants or embedded particles that can either react with the steel surface (i.e., carbon steel or salt) or create crevices and other initiation sites where corrosion can start. The smooth surface, having a low surface profile, is always beneficial to maintain the corrosion resistance of stainless steel. A wide variety of mechanical and chemical methods may use in the processing stage of welded duplex stainless steels. Mechanical grinding can roughen the steel’s surface and also embed unwanted particles, impairing corrosion resistance. For such reasons, after mechanical treatment, passivation is required for maximum corrosion resistance. The chemical surface treatments are aimed to clean the surface of the steel. They may also smooth like electropolishing or roughen like pickling the steel surface or leave it unaffected (passivation) depending on the process. But if carried out properly, they all increase the corrosion resistance of stainless steels. Acknowledgements The paper describes works related to project "Integration of advanced experiments, computation and data for Duplex Stainless Steel joining innovation — i-Weld". This project has received support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement “i-Weld” No 823786”. It was also co-financed by the Polish Ministry of Science and Higher Education under the “PMW” programme in 2020– 2023, contract No. 5107/H2020/2020/2. References [1] J.C. Lippold, D.J. Kotecki, Welding Metallurgy and Weldability of Stainless Steels. Hoboken, NJ: John Wiley & Sons, Inc.; 2005. [2] Duplex Stainless Steels Welding Guidelines, Industeel ArcelorMittal, June 2019. [3] The Avesta Welding Manual: Practice and Products for Stainless Steel Welding. Edition, 3. Publisher, Avesta Welding, 2009, ISBN 9163352338. [4] Australian Stainless Steel Development Association (ASSDA), Stainless Steel Fabrication, 2011, www.assda.asn.au [5] Z. Brytan, Vademecum Stali Nierdzewnej, SSN, Warszawa, 2018, ISBN 978-83-940143-1-5. [6] Huntingdon Fusion Techniques HFT, Heat Tint Recognition Chart for Stainless Steel Welding, www.huntingdonfusion.com