Stainless steel was developed in America over a hundred years ago for use in the weapons industry. Nowadays, stainless steel – based on the combination of iron, chromium, nickel and often also molybdenum – has become an indispensable part of our daily lives. Stainless steel is also used extensively in the food industry. Metallurgist Ko Buijs understands why, but also points out a number of risks.
Ko Buijs knows all there is to know about metals. With his company Innomet, he provides metals-related advice to all kinds of organisations – from hospitals to food manufacturing companies and from large multinationals to small market players. He has received various accolades for his work, including the Future Award. In May 2017 he gave two presentations at surface technology company Rösler about the possible interaction between stainless steel surfaces and food products.
“Simply because it has good cleanability thanks to its high corrosion resistance and smooth surface. This is a result of the protective layer of chromium oxide that covers it. Stainless steel contains sufficient quantities of chromium for it to react with the oxygen in the air to form a chromium oxide ‘skin’. This layer is wafer thin and transparent, but it is enough to prevent water from penetrating into the metal beneath and conversely to avoid metal ions ending up in the food products. Interestingly, if it becomes mechanically damaged this oxide skin can regenerate itself thanks to the presence of oxygen.”
“Well that depends, because stainless steel has to be properly maintained. And it’s important to point out that stainless steel is corrosion-resistant, not corrosion-free. Only precious or noble metals are corrosion-free. Dirt is often a culprit, because it prevents the protective layer from receiving enough oxygen which can result in under-deposit corrosion in that spot. Besides that, the chromium oxide skin is susceptible to certain things such as halogens, which include chlorine, iodine, bromine and fluorine. But brief exposure to chlorine won’t damage the protective layer, plus it can regenerate itself afterwards thanks to oxygen in the air. And after all, you need chlorine compounds to kill bacteria. But long-term exposure to high levels of chlorine, especially at high temperatures, will affect the oxide skin – and we’re seeing this happening increasingly often; as a result of bacteria mutating and becoming more resistant, people are having to use ever-more aggressive cleaning agents and that places considerable strain on the stainless steel.”
“If the oxide skin is damaged, moisture can come into contact with the metal beneath and it will start to corrode, resulting in brown rust-like marks or stains. In some cases, especially in the food industry, the affected machine parts then have to be replaced. Alternatively the corrosion may produce small patches of pitting and indentations in the surface which can harbour bacteria. It’s important to prevent that to avoid food production flows becoming contaminated.”
“Definitely, and there are several ways of doing that. For example, you can use chemicals to complete dissolve a weakened oxide skin and then rebuild it in a process of passivation, although the chemical agents required are aggressive ones. Another option is fine sanding, but that requires a lot of specialist skill. Electropolishing achieves fantastic results in terms of smoothness, but substantial numbers of bacteria can also be left behind during use. It sounds like a contradiction, but if the surface is too smooth and polished then bacteria can actually stick to it really well thanks to negative pressure and adhesion. Nobody wants that, so you need a certain degree of roughness.”
Ko Buijs: ‘There’s no such thing as corrosion-free stainless steel. It’s corrosion-resistant’
“I myself am a big fan of the PureFinish processing technique that Rösler has developed. A watery, abrasive agent is sprayed onto the stainless steel surface under a certain amount of pressure. This achieves the best results in my opinion. Bacteria can be removed from the surface easily and a study by TNO shows that cleaning achieves optimal results compared with other treatment methods. Hence, this is the optimum finish for the stainless steel. Another big advantage is that no chemical agents are required, meaning that this method has no negative impact on the environment.”
“Workbenches and machines have to be cleaned regularly, for one thing. That’s done using a special cleaning agent that also kills off bacteria. This often involves the use of chlorine compounds, in which case it’s important to listen to professional cleaning advice. It’s also important to stick to the specified cleaning time. That’s why things went terribly wrong at an American meat processing company once. The company had a high risk of the presence of Listeria so the machines were foam-washed and the foam contained sodium hypochlorite. Around 15 minutes is generally enough to kill off Listeria bacteria. However, the cleaners themselves had other ideas and they left this corrosive substance on the surface for more than an hour. That destroyed the oxide skin on the stainless steel and caused rust marks on the surface.”
“You can use electrodes to add very tiny quantities of silver and copper ions to the water. The silver weakens the outer membrane of the bacteria, making it easier for the copper to penetrate and act as a biocide to kill them off.”
“Many companies underestimate the risk of microbially induced corrosion (MIC) caused by sulphate-reducing bacteria. As soon as these die off, sulphuric acid can be produced and/or cathodic depolarisation can occur. Sulphuric acid dissolves the oxide skin on the stainless steel, leading to serious corrosion which can contaminate food flows. This process can sometimes happen very quickly; fire-extinguishing pipes in various motorway tunnels have developed leaks within the space of a year. Sulphate-reducing bacteria occur primarily in places with stagnant water, such as fire-extinguishing systems and water reservoirs, so it makes sense to take preventative measures in such situations.”
“For a food manufacturer, it is extremely important to work safely and hygienically. You want to avoid making people ill or causing a product recall. So my advice is to enlist the help of an expert to look at your stainless steel surfaces. The inspection will enable you to assess the current status and tackle any problems before it’s too late. ‘Discover the surface’ is Rӧsler’s motto, and I’m happy to echo that. And if you really want to play it safe, you can consider using titanium for critical components. It’s a maintenance-free material that is not susceptible to MIC or other forms of corrosion. It’s more expensive, but it has a much lower specific weight meaning that the relative price per kilo is not as high as many people think. Whatever you decide on, remember that the surface of the chosen metal always has an impact on the quality of the end product.”
Source: © Ton Kastermans Fotografie
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