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CORROSION Section 2: Basic Corrosion Measurements
By D.H. Lister & W.G. Cook Department of Chemical Engineering University of New Brunswick
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If a piece of metal: corrodes to this: How would we know how much corrosion had occurred? Clearly, we need before and after measurements ……… but of what?
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Size? Mass? Length, breadth and thickness before … What about after?
Volume before, volume after ….. by liquid displacement …. Sensitive enough? Mass? Weigh before, weigh after (if piece small enough for reasonable balance … note that measuring weight losses to a fraction of a mg in a few grams is the norm in corrosion testing). To be of any use, a weight loss measurement has to be scaled to the size of the specimen … corrosion is a surface related phenomenon.
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Because corrosion is a surface phenomenon, weight loss is related to surface area (SA)
For example: a metal “coupon” having a total surface area of 10 cm2 loses 1 mg when exposed to a particular environment for a month; which should be a characteristic amount for that metal in that environment in a month.
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Corrosion engineers traditionally work in units of dm2, thus:
BUT … even though mdms give reasonable numbers for most corrosion situations, the month is a bad unit … Why? So … are often used …. mdd
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If we know the metal density, we can convert the weight loss to “penetration”:
for ρ in g/cm3: Which isn’t a very practical unit.
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However, we note that 1 cm = 104 mm, thus:
Another common unit for corrosion penetraion (in the US) is the mil … or milli-inch per year (=0.001 in/year).
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NOTE: these rates are averaged over the exposure time, and are quoted as if corrosion were constant with time … it often is not … measure rate 1 = measure rate 2 =
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Often, our bit of metal .. after exposure, looks like this … It is covered with scale. How would we assess corrosion under these conditions?
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Scaled (e.g., oxidized) specimens often experience a weight gain … thus, the high-temperature oxidation of iron proceeds … 4Fe + 3O2 → 2Fe2O3 (456)g (456 + 616)g If we know scale composition (e.g., type of oxide) and weight gain, we can calculate the Fe in the oxide and, therefore, the Fe corroded.
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BUT … we can rarely be sure that the scale accounts exactly for the corrosion … some might fall off (spall) … some might have been deposited from the environment … NOW WHAT?
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Consider the scaling process… Weight before = Wo g
Surface area = A dm2 Exposure time = θ days Find condition; weight after = W1 g
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We can only be sure that the remaining metal provides an unequivocal reference, so …
Descale Descaled weight = W2 g
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(divided by θ for rate … mdd)
We also have a measure of the weight of scale … If we assume the scale is composed of Fe2O3, we can calculate the iron in scale as …
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The difference between iron in scale and total amount corroded is either …
Iron released to environment … OR … Iron deposited from environment
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How do we descale? Chemically or electrochemically … usually in weak acids. BUT … such descaling can also dissolve some of the remaining metal, so … how do we account for this? employ an “inhibitor” use a blank …
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A blank is a “bare” specimen put into the descaling solution along with the specimen;
during the descaling, if the blank corrodes by: we apply this blank correction to the specimen measurement DRAWBACKS? DISCUSS EXAMPLE.
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Note: such corrosion measurements and units are useful for general or uniform corrosion but they don’t work for localized corrosion (e.g. pitting or cracking). A component can fail by stress corrosion cracking (SCC) with no detectable weight loss …
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“crack depth” or “pit depth” – usually in mil or mm;
For these situations, we use concepts such as crack propagation rate or pit propagation rate. For example: “percent through-wall” for tubes, pipes, vessels, etc.; “crack depth” or “pit depth” – usually in mil or mm; “propagation rate” – etc.
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Examples of Corrosion General corrosion ….
General corrosion of the reactor vessel head of the Davis-Besse PWR. (note: this exterior general corrosion was indicative of a much more serious local corrosion problem due to a leak in the pressure boundary)
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Classic crevice corrosion
Classic crevice corrosion. In this case caused by a carbon rubber exhaust flapper clamped onto a stainless exhaust flange plate. Notice that the worst corrosion is where the clamp pressed the rubber the tightest.
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Stress Corrosion … Stress corrosion of stainless steel type 304 autoclave. (Mallinckrodt Chemical Works)
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How would you “measure” (estimate) the corrosion of plant components?
For example: pressure vessels or pressure tubes; feeder pipes steam generator tubes; feedwater pipes and heaters; etc. DISCUSS …
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Methods involve … and include … Non-Destructive Evaluation (NDE) or
Non-Destructive Testing (NDT) and include … Ultrasonic Testing (UT); Eddy Current; Acoustic Emission; etc.
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Some useful conversion factors:
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