[ Surface Finishing ] The optimisation of stainless steel surfaces Over the decades researchers have searched for optimal and efficient techniques to adequately recondition the surface of stainless steel after it has been subjected to various processes. This article examines the various processes currently used and the surface conditions achieved. An independent study has shown that water jet blasting may offer the best solution in terms of both the degree of smoothness achieved and the avoidance of using chemical products. By Ko Buijs, Innomet, The Netherlands Welding is one example of a process the development of mechanical grinding polished surfaces have an advantage that results in the need for surface or the use of glass bead blasting. These because the chances of contamination treatment in stainless steels. Tarnishing operations, however, typically provide that may occur will be considerably caused by welding and heat treatments a surface condition which is not suitable smaller. Therefore, the corrosion must be thoroughly removed because for food applications. In time ceramic resistance of a wet blasted surface is such zones and surfaces are microporous bead blasting was developed, which better than that of the original semi- as the darkened areas, making it easy provides a better result as the ceramic finished product, but it is somewhat for aggressive chlorides to penetrate. pellets last much longer than glass. lower than in case of electro-polishing. A normal healthy oxide layer is only Glass beads shatter sooner, causing 10 to 15 nanometres thick and consists sharp edges which are detrimental to the The surface condition of several ultra-thin layers. These surface condition. All these methods are The corrosion resistance of stainless layers will swell during the oxidation designed to treat the material to achieve steel is not only dependent on the process. Due to the mutual differences in acceptable corrosion resistance. type of alloy, but also on its surface expansion coefficient, these layers will condition. For example, a parking pole cause extra microporosity in temperature Polishing constructed of AISI316 stainless steel at variations. Moreover, the underlying More specialised methods are anodic a beach will corrode because of marine metal is more burned chrome than iron, and electro-polishing, which create aerosols. However, this deterioration also resulting in reduced corrosion a particularly smooth, high-quality only occurs on the vertically arranged resistance. In other words, tarnishing oxide layer. In addition, during this grinded pipe while the polished cap caused by welding and/or atmospheric process, more iron atoms disappear of the same alloy remains in excellent glowing should always be removed, than chromium and nickel atoms, which condition. This proves that grinding especially if the area shows signs of results in a better corrosion resistance. has a negative influence on the corrosion afterwards. Reconditioning The positive potential of the oxide corrosion resistance, although this effect aims to obtain sufficient corrosion layer is almost twice as high during decreases if one uses a finer abrasive resistance of the stainless steel surface. electropolishing. This decreases again grain. Pickling is often still an ideal with respect to an as-delivered finish compromise in terms of capability and Pickling and passivation slightly when grinding or glass bead size. Moreover, it is advisable to leave One of the best known surface treatment blasting is applied to the surface. This pickling to specialised companies methods is pickling and passivation, is because of the rough surface that is because the chemicals pose health which has proven to be a good solution created and the negative impact of dirt risks to humans and the environment. over the years. During pickling and deposits which then lead to an ‘under Companies operating pickling lines passivation the damaged, burned, deposit attack’. This is a form of corrosion must comply with stringent rules to and/or contaminated oxide layer is that takes place under dirt deposits, avoid putting their employees at risk of stripped away in order to be built particularly in the presence of chlorides. injury, both internally and externally. up again; in other words passivation Chlorides can penetrate deeper under occurs. Although this method is a good dirt deposits than the relatively large About the author solution to restore weakened chromium oxygen molecules, which eventually Ko Buijs is a recognized oxide film to optimum condition, there compromise the material. Chlorine is metallurgical/corrosion are also drawbacks. Apart from the one of the halogens and these are salt- specialist for stainless steels environmental impact, the process formers. Stainless steel actually needs as well as special metals. He often required the goods to be sent to oxygen to maintain its passivity, but is also a lecturer for several specialised pickling companies, which under dirt deposits there is insufficient organizations such as stainless is disruptive to the internal logistical oxygen present for this to occur. steel associations, Technical High Schools and production flow of the manufacturing Therefore, stainless steel is not innovation centres. He has published over 150 companies. This has prompted maintenance-free and has to be cleaned papers in a number of technical magazines. companies to look for alternatives and regularly. It will become clear that www.stainless-steel-world.net Stainless Steel World July/August 2016 1 [ Surface Finishing ] Figure 4. The right half is treated with the PureFinish process (photo Rösler). be seen schematically in Figure 2, where the degree of ability to clean is displayed in dependence of the roughness of the surface. In other words, the rule “the smoother the surface, the better the ability Figure 1. Pickled stainless steel AISI 316 (V=1500x) whose grain boundaries are affected to clean” does not always seem to be true preventively (source Dockweiler). in practice, when it concerns bacteria and micro-organisms. Figure 3 shows how It will become clear that these measures boundaries provide space for all kinds of this removal of bacteria on a surface takes complicate the business process. contamination and microbial deposition. place schematically. However, the motivation to keep This latter can quickly lead to the A topographical survey by TNO showed products in-house provides benefits dreaded microbially induced corrosion that the glass bead blasting delivers a such as not disturbing the internal (MIC) and other bacterial ‘infections’. particularly rough surface. Such a surface logistics and saving on transportation Therefore, the eye is not always the was found to have an average roughness costs. It is also of great importance that optimal instrument to assess an indicative of about Ra = 1.3 µm. A surface treated staff in the pickling plant are equipped roughness because many rough areas by wet blasting, however, reached a with the right media, parameters, and are not always visible to the naked eye. value of 0.25 and 0.60 µm. inhibitors in order to avoid, for example, Generally, one can argue that a cut over-pickling. A particular risk is that surface is 2.5 to 4 times larger than a Water jet blasting the less resistant grain boundaries can polished surface. It is important that a In recent years the method of water jet be compromised relatively quickly, surface is smooth, but also not too smooth; blasting with an abrasive additive has leading to a roughening of the surface. when the surface roughness is below received quite some attention and it In Figure 1, one can see a clear Ra = 0.2 µm, accelerated bacteriological is expected that this surface treatment preferential degradation of the grain contamination may arise because the will be increasingly used because boundaries in austenitic stainless steel. adhesion forces between the surface and one can achieve interesting benefits. It goes without saying that these grain bacteria have become larger. This can For example, the company Rösler has Figure 2. The cleaning of stainless steel is best done with a roughness of Figure 3. When a surface is too smooth bacteria are difficult to between 0.25 and 0.6 µm (source Leser Hamburg). remove due to vacuum and adhesion (source Leser Hamburg) 2 Stainless Steel World July/August 2016 www.stainless-steel-world.net [ Surface Finishing ] the oxide skin is stripped. Chlorine is one of the halogens or salt formers that want to form metal salts and this is the explanation for the formation of a stain. This then calls for the use of demineralised water. The benefits of this surface treatment are: • Defined, homogeneous and reproducible surface structure which also has a higher overall resistance to corrosion; • High resistance under ‘deposit attack’; • It can replace pickling and passivation which often must be done by specialised companies; Figure 5: Housing of a “rotating clean flow” (source Goudsmit). • Discoloration, coarsening and heavy oxidations caused by the treated parts for the aviation sector is partly explained because the welding process are removed; with water jet blasting with an added roughness is between 0.25 and 0.60 µm • A very easy to clean surface spheroidal abrasive agent for many and that appears to be ideal to obtain that meets sanitary and aseptic decades; for convenience, they call this such results (Figure 2). The European requirements; process – including the medium used - Hygienic Engineering & Design Group • Hardly any tiny spots where food ‘PureFinish’. Customers such as Rolls (EHEDG) demands a roughness of Ra residue can remain behind; Royce and Airbus prefer this surface <= 0.8 µm and the surface amply meets • Shorter cleaning times which treatment. The water with additive these requirements. Figure 7 shows a improves the production yield; forms a suspended medium which is housing of a “rotating clean flow” of • Light pressure build-up on the sprayed at pressure onto the surface. AISI316 stainless steel before and after surface which reduces the risk of This results in an attractive finish and treatment with PureFinish.