Corrosion measurement units

Units of measurement of corrosion rate

In general, to facilitate the study of corrosion problems and also to compare different metals with each other, the best method is to express the speed or rate of the corrosion reaction. Units of measurement of corrosion are used to express the corrosion rate. A suitable criterion for expressing the corrosion rate should have the following conditions:

1. It should be easy to calculate and the probability of error in its calculation is low.
2. It should be used to calculate the rate of various types of corrosion.
3. It should specify the depth of corrosion penetration.
4. If possible, the result of calculating the corrosion rate should be in whole numbers.

The conventional units for expressing the corrosion rate are:

1. Mpy Due to the importance of reducing the thickness of the metal due to corrosion, the corrosion rate is used in terms of thousandths of an inch per year (mpy), that is, how much the thickness of the object has been reduced during one year.

Mr. Fontana stated the following formula in 1945 for calculating the corrosion rate based on the reduction in thickness.

mpy = 534W/DAT

In this relation, W is the weight loss of the sample in milligrams, D is the density of the sample in grams per cubic centimeter, A is the sample area in square inches, and T is the test time in hours.

It should be noted that this relation is valid when the sample surface is completely uniformly corroded and the thickness of the part has decreased by the same amount in all dimensions.

2. mmd To express the corrosion rate, the percentage of weight loss can be expressed in milligrams per square decimeter per day (mmd). This criterion is not very suitable, because in most cases, corrosion products (oxides, metal hydroxides, and deposits) are deposited on the metal surface and cause an increase in the weight of the metal after the corrosion reaction, which prevents the actual corrosion rate from being obtained.

µm/year: Corrosion penetration depth (micrometers per year)

gr/m2.day: Weight loss (grams per square meter per day)

mm/year: Corrosion penetration depth (millimeters per year)

ipy: Corrosion penetration depth (inches per year)

ipmo: Inch per month

If the corrosion rate is less than 0.005 ipy, metals in this group have very good resistance to corrosion. These metals are used to manufacture sensitive parts such as internal valve seats, etc., and if the corrosion rate is between 0.05 ipy and 0.005 ipy, metals in this group are used in cases where they can withstand existing or potential corrosion. For example, they are used in the manufacture of tanks, pipes, and valve bodies. Finally, if the corrosion rate is more than 0.05 ipy, this group of metals is not satisfactory in terms of their resistance to corrosion and it is necessary to change the material of the parts.

In the case of pitting corrosion, there is usually no weight loss or thickness reduction. Instead, for this type of corrosion, the depth of a number of holes is measured per unit area (one square centimeter) and then the average depth is obtained.

Depth of the deepest hole / Average depth of holes = pitting coefficient

If the depth of the hole is more than 25% of the pipe thickness, it is necessary to replace it. If the depth is less than 25% or equal to 25% of the thickness, it can be filled with spot welding and then covered and cathodic protection applied to it.

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