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.

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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

4 36 grit + pickled / 80 grit metre s passivated o r 3 , mi c

a Wire brushed

R 120 grit

220 grit + 2 pickled / Sand blasted 220 grit passivated Roughn ess, e

c Other As heat treated, a 1 f Mechanical pickled Su r

360 grit 0 ???? ???? ??? ??? ??? ???

Corrosion resistance improves as you go to the right of this graph. The graph shows the relative importance of the smoothness of the surface and chemical treatment of the surface. They can be used togther to get the best corrosion resistance. Corrosion resistance improves as you go to the right of this graph. The graph shows the relative importance of the smoothness of the surface and chemical treatment of the surface. Th ey can be used together to get the best corrosion The study reported by G.Coates (Materials Performance August 1990) looked at the effect of various methods of treating an resistance. artificial welding heat tint on a 2B surface. The study reported by G.Coates (Materials Performance August 1990) looked at the effect of various methods of treating an artificial welding heat tint on a 2B surface. p.60 CORROSION & MATERIALS TECHNICAL NOTE

Contrasting features of pickling and passivation

Pickling Passivation Removes both weld oxide and stainless steel Does not remove weld oxides not attack stainless steel Typically hydrofluoric, sulphuric (or in steel mills, Nitric acid or electropolishing. Chelating formulations can hydrochloric) acid have similar effects to nitric acid Removes surface breaking sulphides and carbon steel Removes surface breaking sulphides and, less rapidly, carbon steel smears Changes appearance as it lowers gloss by etching Nitric and chelating treatments should not change appearance. Electropolishing will brighten and slightly smooth it Usually mixed with nitric acid – and with rinsing leaves a Will improve the passive film over a simply pickled one. passive surface

methods by providing an economical Stainless Steel Parts, Equipment Bath composition (acid strength and means of brightening many shapes or and Systems. iron concentration) must be checked forms that cannot readily be finished as extended pickling times or old mechanically. ASTM A967 Standard Practice for solutions tend not to give as good Chemical Passivation Treatments for a result. Electrolytic methods should not be Stainless Steel Parts. specified to remove surface blemishes Dirt and grease will mask the surface such as scratches, burrs, pits, scale ASTM B912 Standard Practice for from treatments outlined above. patterns, forging marks, etc. although Passivation of Stainless Steels using Therefore, the steel surfaces must be they will do so if such defects are very Electropolishing. free of these agents before applying shallow. It is important to realise chemical treatments. that defects initially present on These very useful documents give surfaces may be greatly accentuated detailed recommendations on many Many of the chemical treatments if not treat appropriately. aspects of selection, application and described contain strong acids and evaluation of these treatments and are both handling and PPE requirements Surface condition before highly recommended. must be followed. Before disposal they electropolishing governs the finished will require neutralisation. Check appearance. These processes are also Precautions with your local authority concerning applicable to cast stainless steels. Pickling and passivation use strong the requirements for trade waste, However, the resulting surfaces will acids, and normal precautions for neutralisation and disposal. not be as smooth as those on wrought safety should be followed. Consult materials, unless they are mechanically the Materials Safety Data Sheets and Many of the chemicals described prepared beforehand. product packaging for detailed advice. above will be classified as hazardous substances under State OHS legislation, Most commonly, phosphoric and Note that hydrofluoric acid with implications for purchasing, sulphuric acids are used in conjunction is a schedule 7 poison and in transport, storage and handling. with a high current density to clean concentrations greater than 1% and smooth (by metal removal) requires a specific licence for use. Chemical treatments are useful the surface of the steel. The process tools in cost effectively achieving preferentially attacks peaks and rounds For chemical processes that etch the peak performance with stainless valleys on the surface and raises the stainless steel, reaction times will steels. With appropriate training, proportion of chromium at the surface. increase with increasing grade. hazards associated with their use can be managed. The technique can have a substantial More care is required with ‘free effect on the appearance, increasing machining’ grades and these will More details on chemical handling lustre and brightness while only usually require substantially less and disposal are given in suppliers' reducing the measured roughness by aggressive chemicals. The sulphur datasheets and Trade Waste about 30%. addition in these steels means they and/or Environmental legislation are readily attacked by chemical and/or regulations. Standards treatments. Care is also required when The four categories of chemical treating martensitic or low chromium Extract from the Pickling and treatment are detailed in a number of ferritic stainless steels. Passivation section of the Australian standards, but the most commonly Stainless Reference Manual (2012) used are: Detailed recommendations for each published by Australian Stainless Steel grade of stainless steel are given in Development Association (ASSDA) ASTM A380 Standard Practice for the ASTM Standards. Cleaning, Descaling and Passivation of

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