TECHNICAL NOTE Pickling and Passivation The chemicals used in pickling and passivation are acidic and must be handled with due OHS and environmental precautions as well as neutralisation and suitable disposal of waste. In addition, hydrofluoric acid n(HF) is a schedule 7 poison. Stainless steel can corrode in service if there is contamination of the surface. Chemical treatments can be used to improve the corrosion performance of the steel, and hence its appearance in service. Both pickling and passivation are inclusions in the surface of these Electropolishing - electrochemical chemical treatments applied to the products becomes more important. treatments that remove impurities and surface of stainless steel to remove have the added benefit of smoothing contaminants and assist the formation Generally mill finishes for flat and brightening the surfaces. of a continuous chromium-oxide, products (sheet, plate and strip) will be passive film. Surfaces must be free smoother as their thickness decreases. Electropolishing is also an of grease and dirt before using these electrochemical process. It removes acid treatments. A No. 1 (HRAP) finish on a thick gross weld scale and heat tint but is plate may have dimples or other not intended to brighten or smooth Stainless steels resist corrosion best imperfections and a surface roughness the surface. if they are clean and smooth. Clean of 5 to 6 micrometres Ra. means being free of contaminants on Both pickling and passivation solutions or in the surface that can either react A typical 2B cold rolled finish on can employ dangerous acids that can with the steel (like carbon steel or salt) 1.7mm thick sheet might have a damage both the operator and the or that create crevices or other initiation surface roughness of 0.2 micrometres environment if not handled correctly. points where corrosion can start. Ra or better. Pickling acids for stainless steel are Smooth means having a low surface highly corrosive to carbon steel. profile at the ‘micro’ level. Mechanically New surfaces will be created during abrading the surface can roughen the fabrication processes (eg. cutting, It is essential that all acids are steel’s surface and may also embed bending, welding and polishing). thoroughly removed by rinsing the unwanted particles. After mechanical component after completing the cleaning, passivation is required for The corrosion performance of the new process. Residual hydrofluoric acid will maximum corrosion resistance. surfaces will generally be lower than initiate pitting corrosion. the mill supplied product for a number The common feature of chemical of reasons: the surface is rougher; If there are crevices, it may be of treatments is that they all clean the sulphide inclusions sitting just under benefit to neutralise the acid with an surface of the steel. They may also the surface have been exposed; or mild alkali before the final rinsing step. smooth or roughen the steel surface, steel tooling contamination may or leave it unaffected depending on have occurred. Pickling which process is chosen. But if carried Pickling is the removal of any high out properly, they all increase the Chemical treatments that are correctly temperature scale and any adjacent corrosion resistance. performed can clean the surface and low chromium layer of metal from ensure the best possible corrosion the surface of stainless steel by ASTM A380 Standard Practice for performance. chemical means. Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment Chemical Surface Treatments Where the steel has been heated by and Systems is a valuable source These can be grouped into welding, heat treatment or grinding of information on pickling and four categories: to the point where a coloured oxide passivation treatments. Other sources layer can be seen, there is a chromium of information may be obtained by Pickling - acids that remove impurities depleted layer on the surface of the contacting ASSDA. (including high temperature scale steel underneath the oxide layer. from welding or heat treatment) and Stainless Steel Products etch the steel surface. 'Pickling' means The lower chromium content causes During steel making, sulphur in the some of the stainless steel surface lower corrosion resistance. To steel is controlled to very low levels. is removed. restore the best corrosion resistant But even at these levels sulphide performance, the damaged metal layer particles are left in the steel, and can Passivation - oxidising acids or must be removed, exposing a fully become points of corrosion attack. chemicals which remove impurities alloyed stainless steel surface. They can be removed by chemical and enhance the chromium level on surface treatment. the surface. Pickling time is longer with higher grades, lower temperatures or more Most bar products are slightly higher Chelating agents - chemicals that can severe heat tint, which is usually in sulphur for easier machining, remove surface contaminants. caused by poor gas purging. so chemical treatment to remove p.58 CORROSION & MATERIALS TECHNICAL NOTE Mechanical removal alone may leave abrasive or other particles embedded (interfering with corrosion performance) or may be impractical. If mechanical means are used, chemical passivation will be required for maximum corrosion resistance. Procedures incorporating pickling solutions, such as a mixture of hydrofluoric (HF) and nitric (HNO3) acids, remove the scale and the underlying chromium depleted layer and restore the corrosion resistance. Mixtures of HF and HNO3 are the most common and are generally the most effective. Acids are available as a bath, a gel or a paste. Commercially available mixtures contain up to about 25% HNO3 and 8% HF. These chemicals etch the stainless steel which can roughen and dull the surface. Pickling solutions also remove embedded contaminants such as carbon steel and ferrous and ferric oxide particles. Pickling solutions other than mixtures of HNO3 and HF acids exist and can be used for specialised applications. They tend to be slower. Pickling pastes, where the solution is mixed with an inert carrier, are commonly used to treat selected areas such as welds. Pickling involves metal removal and a change or dulling in the visual brightness of the metal. Procedures involving electropolishing are a useful alternative or additional treatment following pickling. Metal removal is achieved and usually results in a bright, smooth and more highly corrosion resistant finish. Passivation Passivation is the treatment of the surface of stainless steels, often with acid solutions (or gels), to remove contaminants and promote the formation of the passive film on a surface that was freshly created, eg. through grinding, machining or mechanical damage. Passivation works by dissolving any carbon steel contamination from the surface of the stainless steel, and by dissolving out sulphide inclusions breaking the surface. Nitric acid (HNO3) may also enrich the proportion of chromium at the surface. Some chelants also claim to do this. Example of a good post-weld practice. August 2016 www.corrosion.com.au p.59 TECHNICAL NOTE Common passivation treatments Chelants resistance. Note that a pickled or include HNO3 solutions or pastes Chelants have chemical 'claws' designed passivated rough, matt 36 grit finish which will clean the steel surface free to selectively clean the surface. could have better corrosion resistance of iron contaminants. than an untreated 220 grit finish. The The carboxylic acid group -COOH is study reported by G.Coates (Materials Formulations contain up to about the basis for many chelants which are Performance, August 1990) showed 30% HNO3 and may also contain used in cleaners, water softening and the effect of mechanical and chemical other oxidisers such as sodium lubricants. The pH and temperature treatments of furnace generated heat dichromate. When used correctly, an must be correct for the chelant to do its tint on a high molybdenum grade HNO3 treatment should not affect the job. Chemicals containing these COOH of 316. appearance of the steel. 'claws' include citric and oxalic acids and EDTA. Citric based formulators are Electropolishing Passivation does not usually result in most used as they are not toxic. Electropolishing of stainless steel is a a marked change in appearance of the method of imparting brilliance to its steel surface, although mirror polished Turbulent rinsing of pipes and vessels surface by removal of a thin surface surfaces should be tested first. afterwards is important. layer, especially the peaks. The corrosion resistance of the Cleaning by chelating agents tends to Studies have also shown that stainless steel is affected by the be based on proprietary knowledge and electropolishing benefits corrosion roughness of the surface after systems, and is less standardised than resistance. The work to be polished is polishing, with a marked decrease in the other methods described. the anode in an electrochemical cell the corrosion resistance as the surface The successful use of these systems containing a suitable electrolyte. roughness increases above a Ra value needs to be established on a case-by- of about 0.5 micrometres. This roughly case basis. The process may be considered the corresponds to the surface produced opposite of electroplating and is an by grinding with 320 grit abrasives as Chemical vs Mechanical important production tool in the discussed below. Treatment fabrication of the stainless steels, along with mechanical polishing processes. Either passivation or electropolishing The figure below shows the relative can be used to improve the corrosion importance of the surface smoothness Generally speaking, electropolishing resistance of polished surfaces. and chemical treatment on corrosion supplements the mechanical polishing Effect of chemical and mechanical treatments on corrosion resistance 5 Belt Ground Chemical 36 grit Treatment s 4 36 grit + pickled / 80 grit metre passivated o r c 3 , mi a Wire brushed R 120 grit ess, 220 grit + 2 pickled / Sand blasted 220 grit passivated Roughn e c Other As heat treated, a 1 f r Mechanical pickled Su 360 grit 0 ???? ???? ??? ??? ??? ??? Corrosion resistance improves as you go to the right of this graph.