Q: Our restaurant adopted RO water treatment technology to improve taste and remove harsh minerals, chemicals and odor from our municipality’s water. We were told that doing this would also greatly benefit our stainless steel equipment by reducing scale/corrosion, however, we are still seeing corrosion in some of our equipment. Can you tell us why?
A: Let’s begin by establishing the difference between scale and corrosion.
Scale: Scale occurs when tap water evaporates and deposits calcium, magnesium and iron oxides on the surface of stainless steel. This is why you were told that the use of soft, deionized, and RO water, which removes calcium, magnesium and iron, would mitigate the formation of scale. This is correct.
Corrosion: Corrosion on stainless steel occurs when dissolved oxygen in water (soft or hard) comes in contact with surface iron on the stainless. The surface iron occurs due to a loss of the passivation layer (chromium oxide) on the stainless. Passivation can be lost, and iron exposed, when there are scratches, abrasions, carbon buildup or a manufactured flaw in the stainless passivation layer. Once there is a degraded passive layer (with or without scale present), if water comes in contact with that area, corrosion will start.
Next up, it’s important to understand the role scale plays in stainless steel corrosion. Scale contributes to, but does not directly cause, corrosion. Scale buildup provides nooks and crannies for organic and food residues, which can contribute to enriching the carbon content on the surface of the steel. This is turn can further degrade the passive layer. Scale also contributes to corrosion by providing a rough, porous surface that holds water with dissolved oxygen. To the extent that there is any degradation of the passive layer under the scale buildup, corrosion will be more likely to start under the scale since it is holding water next to the degraded passive layer.
Once corrosion starts, it tends to be auto-catalytic, which means that it will spread. Using purified water will reduce scale build up, which can help reduce the sites where corrosion is likely to start, but it will not altogether eliminate corrosion.
One way to combat this cycle is to use acidic cleaners like Citranox® Liquid Acid Cleaner and Citrajet® Low Foaming Acid Cleaner from Alconox, Inc. These food grade deep cleaners remove grease, eradicate scale and rust, and leave behind the conditions necessary for the stainless to repassivate.