General Corrosion

General corrosion can be thought of as corrosion that proceeds at about the same rate over the entire metal surface that is exposed to the environment. The resulting surface can have areas of greater and lesser penetration such that the maximum penetration can be several times the minimum penetration. The resulting surface can have a somewhat mottled appearance. Uniform corrosion is a special case of general corrosion in which the corrosion rate at each point on the surface is the same. The resulting surface tends to have a uniformly smooth appearance. In electrochemical terms, each point on the surface acts simultaneously as an anode and a cathode.

An example of general corrosion is shown in this picture.

The above picture shows that general corrosion can lead to a surface with bumps and valleys indicating areas of greater and lesser attack. This appearance should not be mistaken for Pitting Corrosion.

Examples of corrosion coupons exhibiting general corrosion are shown in these pictures:

The first example shows a general darkening of the surface that is fairly uniformly distributed across the surface. The weld overlay is etched uniformly as well. In this case, the loss of metal from the weld is about the same as from the base metal so penetration rate as estimated from mass loss (measured loss in weight) most likely pertains to both the weld and the base metal. If weld etch had been extreme so that much more metal was lost from the weld than the base metal (or vice versa), penetration rates estimated from change in weight of the entire specimen would have been in error. Corrosion rates from the weld and base metal would have to have been measured separately. The second example shows some stains as well as some mottling of the surface. Both cases are examples of general corrosion.

Corrosion Rate Estimation

Mass Loss (Change in Weight)

The most common method for estimating a corrosion rate from mass loss is to weigh the corroding sample before and after exposure and divide by the total exposed area and the total exposure time making sure that appropriate conversion constants are used to get the rate in the required units. The method can be represented by the following equation:

In the above equation

R is the penetration (corrosion) rate (e.g. mm/y)
A is the exposed area (e.g. cm²)
m_a and m_b are initial and final masses (e.g. gm)
Δ t is the total exposure time (e.g. hr)
ρ is the alloy density (e.g. gm/cm³)
K is a constant for unit conversion

To provide minimum uncertainty in the corrosion rate, this method implicitly assumes:

the corrosion rate does not vary with exposure time
the area does not change as mass is lost to corrosion
the projected and actual surface areas are the same
the penetration rate is uniform over the entire surface
the weight is unaffected by corrosion product removal

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))and http://www.argentumsolutions.com/tutorials/calculator_tutorialpg2.html discuss these points in more detail. The web site http://www.argentumsolutions.com/calculator.html also provides an on-line calculator for determining the corrosion rate from exposed rectangular corrosion coupons and wires.

Corrosion coupons are sometimes used to estimate residual life or the increased wall thickness that should be provided to ensure the desired equipment life. The above equation is often used to provide the corrosion rate and from that the total expected penetration during the life of the equipment. The greater the uniformity in corrosion rate across the surface, the better that method works. The question is how to make this estimate in the presence of areas of greater and lesser attack as can happen with general corrosion. The issue is that the average penetration rate and the maximum penetration rate can differ by factors of 2, 4 or more. The maximum penetration rate determines equipment life since the region suffering that rate of attack would be the first to fail. One approach is to examine the surface of the corrosion coupon and estimate the localized penetration using either the focus on a microscope or a profilometer. While not perfect, those approaches would at least provide a better estimate than the average corrosion rate. To be conservative, additional thickness can be added.

Corrosion Current

Appropriate use of electrochemical measurements such as the Polarization Resistance Technique, Electrochemical Impedance Technology, Tafel extrapolation, and Galvanic Corrosion testing ultimately results in the estimation of a corrosion current density. This current is a measure of the corrosion rate. In many practical situations, this number needs to be transformed into a penetration rate to make a practical prediction in industry or to enable comparison with other mass loss derived coupon immersion results. The corrosion rate as a penetration rate is calculated from the corrosion current by the following equation:

In the above equation

CR = corrosion rate as a penetration rate (mil/y or mm/y depending on K)
K = constant for converting units
i_cor = corrosion current (amp/cm² or μamp/cm²)
ρ = alloy or metal density (gm/cm³)
EW = alloy equivalent weight (gm/equivalent)

The alloy equivalent weight is calculated by the following equation:

In the above equation

EW = Equivalent weight (gm/equivalent)
n_i = valence of alloying element "i" (equivalent/mole)
f_i = mass fraction of alloying element "i"
A_i = atomic mass of alloying element "i" (gm/mole)

The summation is run over all elements that participate in the reaction. Elements that have a mass fraction of less than 0.01 (less than 1% by weight) can usually be ignored in the calculation. The web site http://www.argentumsolutions.com/calculator.htmlcontains an on-line equivalent weight estimator and an on-line calculator for converting corrosion current to a penetration rate.

As in the case of penetration rate from mass loss, the penetration rate estimated from corrosion current is the average penetration rate across the surface. For that value to apply at all points requires that the corrosion rate be uniform across the surface. The effects of non-uniformity in general corrosion that affect corrosion rate estimates from corrosion coupons also affect corrosion rate estimates from corrosion current. The issue is that the average penetration rate and the maximum penetration rate can differ by factors of 2, 4 or more. The maximum penetration rate dictates equipment life since the region suffering that rate of attack would be the first to fail. Further information on making this calculation can be found at http://www.argentumsolutions.com/tutorials/calculator_tutorialpg4.html. ASTM Standard G-102 "Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements" available from ASTM International provides guidelines for estimating corrosion rates from the polarization resistance technique. Additional information on the assumptions behind corrosion rate estimation from electrochemical techniques can be found at http://www.argentumsolutions.com and the references contained in the various tutorials on that site.

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