Why does Hevasure monitor crevice corrosion?
Thursday, 26th April 2018
As I mentioned in my last blog, Hevasure has developed a couple of novel sensors to help monitor the conditions within HVAC systems. Previously I discussed the importance of measuring galvanic currents to check on general corrosion and the effectiveness of inhibitors at passivating metal surfaces. In this blog I want to explain why measuring crevice corrosion in systems is also critical and how we do this with our patented technology.
Firstly, it is necessary to understand what is meant by crevice corrosion and why this occurs. If you think about where rust usually forms first on steel surfaces you will probably have observed that is seems to be located in joints, weld seams and other sharp bends. Typical examples would be the inner crannies of old car doors and bonnets. In heating systems, crevice corrosion tends to occur at weld seems in radiators, valves and pumps. This is because crevices create a localised environment where oxygenation levels differ from adjacent metal surfaces. On a microscopic-scale, the small area within the crevice tip becomes depleted of oxygen compared to the surrounding metal and the resulting oxygen concentration gradient causes a corrosion cell to be set-up. The tip of the crevice becomes anodic and corrodes preferentially. Obviously, from this description of the mechanism, you can see that oxygen must be present in the first place for crevice corrosion to occur.
You may be tempted to think that this is just an academic point, but unfortunately, it isn’t. Crevice corrosion is all too common and is usually the first sign of an unhealthy system. Crevice corrosion often results in rapid pin-holing where pipes join or at weld seems in radiators, valves and other HVAC components.
Until now it hasn’t been possible to buy a sensor capable of detecting the onset of crevice corrosion. That is why Hevasure has developed and patented such a device to add to the full suite of sensors used in our revolutionary Hevasure monitoring system. In this device (figure 1) we wrap fine wires of different metals in crevices and continually measure the resistance. If and when crevice corrosion starts to occur we can readily detect it and issue alerts before significant damage occurs. We recently came across such an occurrence in a LTHW system and were able to show that slight crevice corrosion had indeed started in connections between thin walled pipes. Fortunately our device had picked-up the onset of the crevice corrosion before significant damage had occurred and we were able to intervene to prevent any further deterioration.
Please contact us if you wish to find out more about crevice corrosion or our unique monitoring services.