Hi Prof Crisi
In design circles, the corrosion terms are much more exact, as these are actually related to the expected wall loss per year, multiplied by the design life of the equipment. Corrosion loss figures related to the "terms" such as those shown in your post, then make sense from a design perspective. To see if this is helpfull from an inspection perspactive, let us "work backwards" while making some assumptions. (Purely intellectual here, not based on any codes or standards!)
The first assumption relates to the design life of the equipment. While this can vary widely where extreme conditions need to be handled, but in most petro-chem applications, the design life would be around 25years. The second assumption is that we are looking at a general corrosion mechanism, not localized. So:
1.5mm / 25y = 0.06mm loss per year
3.0mm / 25y = 0.12mm loss per year
6.0mm / 25y = 0.24mm loss per year
If we further assume (third assumption) that any corrosion products formed is not removed, but stays in place as an oxide scale, then there is a rough rule of thumb that for every mm of wall loss, the oxide build-up is 7mm thick. (Oxide scale on steel.) We can make a further assumption (fourth assumption) that the inspection interval of the equipment is 2 yearly. (Based on the legislated inspection intervals in some jurisdictions, unless RBI studies show it is not necessary.) So the oxide scale thickness for the differing corrosion terms after this 2 year interval will be:
Light = 0.06mm/y = 0.84mm scale over 2 years. (Scale this thickness will tend to be relatively tightly adherent and hard, making a near continuous film, possibly with "steps")
Moderate = 0.12mm/y = 1.68mm scale over 2 years. (Scale this thickness will tend to be visible as a number of different layers or steps.)
Severe = 0.24mm/y = 3.36mm scale over 2 years. (Scale this thick will tend to be relatively flaky and look rather bad.)
We have obviously made a whole lot of assumptions, but many of these will hold in typically stagnant areas in equipment subjected to regular inspections. I believe that quite probably the terms the inspector might use for the quantities of corrosion product as given above, may very well be in line with the design intent, so this may be quite instructive for inspectors. Obviously a vessel with a "light corrosion" rate of 0.06mm/y, if only inspected every 6 years (Totally plausible) will have a corrosion product of 2.5mm thick, which would almost certainly be described as "severe" by most inspectors. This shows us that the terms are very relative when used by the inspector in isolation. It needs to be backed up by other data such as thickness testing. Obviously a surface can be subjected to erosive effects from the fluid, which would remove the corrosion products, and the result is that the inspector will see no corrosion at all. Again we need more data. Generally though, corrosion is rarely totally even. As corrosion products build up, they actually have a temporary protective effect, but due to their inherent stresses, they spall off, leaving certain areas exposed to experience the previous corrosion rates. This sort of mechanism then leads to the corroded surface having a "step like" appearance, or laking or pitting can also result. This makes the corrosion more obvious, and will lead the inspector to ask more questions.
Thanks Prof Crisi, I think this is not a bad starting point from which to think of the corrosion terms. (Nothing exact, but interesting.) - Now come on, I know there are a whole lot of you that do not agree at all, and think this exercise is full of holes, so let us hear!!!
Regards
Niekie