In the food industry, there are two common materials that can be corroded under the wrong conditions, these being stainless steel and aluminium. With both materials, corrosion happens when the protective oxide layer is compromised, and this can happen in the following ways:
Aluminium – is classified as a “soft metal” and is vulnerable to solutions that are too acidic or alkaline (below 4 or above 9 pH), as these will strip the aluminium oxide layer off and leave it open to rapid corrosion. Also, sitting permanently in water can have the same effect over time, as it needs exposure to air to re-oxygenate and form a preserving layer again.
Normally neutral or near-neutral detergents are recommended to clean aluminium components, but if the soiling is tenacious or burnt on, these will not normally suffice. In these situations, it is best to use a highly alkaline detergent to attack the soiling but ensure that it has adequate silicate component or corrosion inhibitors to prevent it attacking the aluminium substrate under the soiling – your chemical supplier should be able to advise on this.
In all instances, you should leave the chemical on only long enough to loosen the soiling before rinsing off and allow the aluminium surface to dry completely before re-using.
Stainless Steel – Both of the common grades of stainless steel used in the food industry – 304 and 316 – are far more resistant than aluminium to extremes of pH and can safely tolerate very alkaline and acidic fluids. However, if these grades are exposed to high heat at the same time as high concentrations of strong acid or alkali solutions, they can quickly become damaged.
If your process routinely exposes your s/s equipment to chlorides, in the form of brine solutions or liquid chlorine for example, then you are best to choose 316 grade equipment as this is optimised to resist chloride corrosion
If chlorides are not rinsed properly from equipment, they can lodge in crevices or weld seams or areas of low-flow in pipework and cause what is known as pin-hole corrosion. Pin-hole corrosion is different and more serious than what is known as general corrosion (where a larger surface area loses x amount of microns from the surface over a given time period), because it can quickly work through the entire thickness of the steel in a very defined area and cause leaks in pipes and compromise the integrity of the equipment.
There is another type of corrosion of s/s equipment in the food industry which is known as microbial or biofilm corrosion. This can happen when cleaning regimes are not adequate, and a biofilm establishes itself inside a pipe for example. The corrosion happens underneath the EPS layer of the biofilm and can be caused by three different factors – the biofilm layer excludes any oxygen reaching the surface of the steel, so it cannot replenish its chromium oxide micro-layer, the bacteria within the biofilm excrete acids which concentrate in a localised area and remove the micro-layer, and chlorides can become trapped in the biofilm matrix and start pin-hole corrosion, as discussed previously.