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TUTORIAL ON CORCALC (Corrosion Rate Calculator)

David C. Silverman


Table of Contents

Overview of Tutorial
Corrosion Rate From Coupon Mass Loss
Corrosion Rate From Wire
Corrosion Rate From Corrosion Current
        

Corrosion Rate From Coupon Mass Loss

Perhaps the most common method for estimating if a material is compatible with an environment is to immerse that material in the environment for a predetermined time period and to evaluate its performance after removal from the environment. The prediction is only as good as the ability of the test to duplicate the actual environment. The resources suggested at the end of the Overview of Tutorial
section can be consulted for further information on how this test can be modified to duplicate complex environments more accurately.

One type of specimen commonly used for such immersion is a rectangular shaped specimen, often between 0.125 inch and 0.25 inch (3.18 mm to 6.35 mm) thick. Such specimens are often called "corrosion coupons". When metals or alloys are being evaluated, the change in mass is obtained by weighing the specimen before and after exposure and converting that mass change during the exposure period to a penetration rate. Very often, a weld is made along the center of one side so as to be able to assess selected attack of the weld. The standard calculation does not single out such selective attack.

The calculator uses the following equation to estimate the corrosion rate:

                                                                        (1)


where:
                         R = penetration rate (mil/y or mm/y)
                         mb = mass before exposure (gram)
                         ma = mass after exposure (gram)
                         A = total exposed surface area (in2 or mm2)
                         Δt = total exposure time (hours)
                         ρ = density (g/cm3)
                         K = constant for unit conversion
The units are those most commonly used and are those assumed for this calculator. The information required to use the calculator for this type of corrosion coupon is:
  • Length (inch or millimeter)
  • Width (inch or millimeter)
  • Height (inch or millimeter)
  • Diameter of bolt hole through coupon (inch or millimeter)
  • Diameter of electrical isolation washers (inch or millimeter)
  • Mass before exposure (gram)
  • Mass after exposure (gram)
  • Exposure time (hour)
  • Density (gram/cm3)
The user types these values into the spaces provided in the calculator and hits "Run Calculator" for the answer which is presented in units of mils per year and millimeters per year. Note that the area is corrected for the presence of either an exposed bolt hole or two electrical isolation or crevice forming washers. The calculator requires an exposed bolt hole to mean that electrical isolation or crevice forming washers have not been used. Similarly, the use of electrical isolation or crevice forming washers means that the bolt hole is not exposed and can be assumed to be absent. A zero must be entered for the diameter of the bolt hole if isolation washers are present or a zero must be entered for the diameter of the isolation washers if the bolt hole is exposed (e.g. coupons held by a "string"). If neither is present, a zero must be entered for both diameters. In addition, two identical isolation washers are assumed if that diameter is greater than zero.

This calculation makes a number of assumptions about the experiment that are sometimes overlooked by the practitioner:
  1. The penetration is uniform or even across the specimen surface. No areas of greater and lesser attack are observed. This assumption is very rarely fully obeyed even when the corrosion mechanism is considered "general" corrosion. The implication is that the maximum penetration rate may be several times the average penetration rate.
  2. No localized attack occurs. In the extreme, a single pit may occur on the surface surrounded by a large area that is virtually uncorroded. In this case, the penetration rate as estimated from the average mass loss has no relationship to the maximum penetration rate as dictated by the propagation rate of the pit. The maximum penetration rate not the average penetration rate may imply potential failure whereas the average penetration rate would not. Coupons should always be examined under a stereomicroscope at 10X to 30X magnification. Results from equation (1) should be used with extreme caution when localized corrosion is observed.
  3. The projected and average surface areas are the same. The actual surface area is determined by the contours of the local surface. A smooth surface will have a much smaller difference between the "peaks" and "valleys" than a rough surface. The surface area usually used for the corrosion rate calculation in equation (1) is most often that obtained with callipers, the projected surface area. Differences between actual and projected areas tend to be ignored. Often, the rougher the surface, the larger the difference between the projected and actual surface areas.
  4. Areas (dimensions) are unchanged during exposure. The usual procedure is to measure the corrosion coupons before exposure and assume those dimensions when estimating the surface area. This assumption, while often being reasonable, still imparts an inherent error in the calculation. The assumption may break down if the corrosion rate is high or the exposure time is very long accompanied by a moderate corrosion rate.
  5. Weights are unaffected by corrosion product removal procedures. Sometimes corrosion products can be so tenacious that their removal becomes difficult. Care must be taken that only the corrosion products or loosely adherent oxides are removed and the good underlying metal is left intact. ASTM G-1, "Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens" is a good source of information on this subject.
  6. The penetration rate does not vary during the exposure. The time used in the calculation is usually the total exposure time for the given specimen. Corrosion rates are sometimes not constant. They change as corrosion proceeds, decreasing in some cases, increasing in others. For example, sometimes the corrosion rate of stainless steel may be somewhat higher initially as the iron in the surface selectively corrodes leaving a chromium enriched surface behind. The corrosion rate then would decrease as the chromium enriched surface begins to impart more passivity. So-called "planned interval tests" can be used in extreme cases. But, equation (1) can be used for each interval.
Even assuming that the above criteria are fulfilled, errors can still be propagated because of the uncertainty in the measurement of time, mass, and dimensions. A recent paper (R. A. Freeman and D. C. Silverman, "Error Propagation in Coupon Immersion Tests", Corrosion, Vol. 48, No. 6, p. 463 (1992))1    (194k) discusses the implications in detail. The key points from that paper are:
  • The ASTM "rule-of-thumb" that for low to moderate corrosion rates the test duration in hours should equal 2000 divided by the corrosion rate in mpy (see ASTM G-31 "Standard Practice for Laboratory Immersion Corrosion Testing of Metals") seems to provide a very low standard error for corrosion rates between 0.1 and 10 mpy. Some relaxation from that conservative estimate would likely not lead to an exorbitant increase in the error.
  • The above "rule-of-thumb" allows an uncertainty of 30 minutes in time measurement to have little effect on the error in corrosion rate. In fact, in many instances, if all other uncertainties are nil, reporting the test time to the nearest 30 minutes can be adequate if the test lasts 7 or 30 days.
  • Coupons should always be weighed on a balance that provides accuracy to 0.0001 grams (0.1 mg).
  • An error that is a fairly large percentage of the absolute corrosion rate (as high as 400%) can result when any of the following occur: (1) corrosion rates are low (virtual contamination rates), (2) the test duration is short (e.g. less than one week), (3) the balance is not calibrated or the accuracy is not 0.0001g, and (4) calipers are not used to measure dimensions. The above paper provides more detail.
  • Long test times as might occur for immersion in an operating plant environment will compensate for a higher level of uncertainty in these measurements.
  • The prediction is only as good as the ability of the test to duplicate the actual environment.



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1 © NACE International publication and year shown in citation above. All rights reserved. Displayed with permission from NACE International, Houston, TX (http://www.nace.org). Published in Corrosion, in the month and year shown in the citation above.





David C. Silverman, Ph.D. - Primary Consultant
E-Mail:     dcsilverman@argentumsolutions.com
Phone:     314-576-3586
Fax:         314-754-9825
Address:   The Argentum House
                14314 Strawbridge Ct.
                Chesterfield, MO 63017