Corrosion Rate Measurement.pdf 2u4o1f

Corrosion Basics

Corrosion Rate Measurement

CORROSION RATE MEASUREMENT WEIGHT LOSS METHOD

specimen during the corrosion test.

The

The rate of uniform corrosion can be

conversion from other units to obtain mils

measured using corrosion coupon testing by

per year is given in Table 2. As per ASTM

weight

Coupon

G31 calculating corrosion rates requires

corrosion testing is predominantly designed

several pieces of information and several

to investigate uniform corrosion.

ASTM

assumptions; (i) the use of corrosion rates

Designation: G31 gives a definite guideline

implies that all mass loss has been due to

for carrying out such an experiment. This

uniform corrosion and not due to localized

practice describes accepted procedures,

corrosion, (ii) the use of corrosion rates also

which

loss

measurement.

specimen

preparation,

implies that the material has not been

conditions,

method

of

internally attacked as by dezincification or

cleaning specimens, evaluation of results,

intergranular corrosion and, (iii) internal

calculation and reporting of corrosion rates.

attack can be expressed as corrosion rates

A good corrosion rate expression should

if desired.

involve (i) familiar units, (ii) easy calculation

calculation must not be based on weight

with minimum opportunity for error, (iii)

loss (except in qualification test such as

ready conversion to life in years, (iv)

practice A 262), which is usually small but

penetration and (v) whole numbers without

on microsections, which show depth of

cumbersome decimals.

attack.

includes

apparatus,

test

However, in such a case the

Corrosion rates have been expressed

Assuming that localized or internal corrosion

in a variety of ways in the literature; such as

is not present, the average corrosion rate

percent weight loss, milligram per square

can

centimeter per day, and grams per square

equations:

inch per hour.

These do not express

be

calculated

by

the

following

Corrosion rate = (K x W) / (A x T x D)

corrosion resistance in of penetration. The expression mils per year is the most

Where K is a constant; T the time of

desirable way of expressing corrosion rate.

exposure in hours to the nearest 0.01 h; A

This expression is readily calculated from

the area in cm2 to nearest 0.01 cm2; W the

weight loss of the metal or the alloy

mass loss in g to nearest 1 mg (corrected

Pressure Vessel Engineering

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Corrosion Basics

Corrosion Rate Measurement

for any loss during cleaning) and D the

T, A, W and D from Table 3, the corrosion

density in g/cm3.

rate can be calculated in variety of units with

Many

different

units

are

used

to

appropriate value of K given in Table 3.

express corrosion rates. Using the units for

Table 2: Conversion from other corrosion rate units to obtain mills per year Unit to be Converted Inches per year

Multiplier 1000

Inches per month

12.1000

Millimeters per year

39.4

Micrometers per year

0.039

Milligrams per square Decimeters per day (mdd)

1.44/density

Grams per square meter per day

14.4/density

Table 3: Corrosion rate units with appropriate value of K Corrosion Rate Units Desired

Constant (K) in corrosion Rate Equation

Mils per year (mpy)

3.45 x 106

Inches per year (ipy)

3.45 x 103

Inches per months (imp)

2.87 x 102

Millimeters per year (mm/y

8.76 x 104

Micrometers per year (µm/y)

8.76 x 107

Picometers per second (pm/s)

2.78 x 106

Grams per square per hour (g/m2h)

1.00 x 104 x D4

Milligrams per square Decimeter per day (mdd)

2.40 x 106 x D4

Micrograms per square meter

2.78 x 106 x D4

Per second (µg/m2 s)

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Corrosion Basics

Corrosion Rate Measurement

A

Density is not needed to calculate the corrosion rate in these units. The Density in the constant K cancels out the density in the corrosion rate equation.

Monograph for mpy, ipy, ipm and mdd A rapid and ready conversion for

This permits conversion of mills per

several corrosion rates can be made by

year, inches per year, inches per month and

means of a monograph shown above.

milligrams per square decimeter per day

Mathematical

are

not

(mdd) from one to another. The first three

necessary, and the accuracy is good.

The

names are directly converted on the scale

computations

monograph is particularly helpful when data in milligrams per square decimeter per day are encountered.

A. These are then converted to mdd by means of C scale and the B scale for

Pressure Vessel Engineering

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Corrosion Basics

Corrosion Rate Measurement

The mdd does not consider or

the equipment under test, is exposed to the

include the density or type of material

process stream. The electrical resistance of

involved.

this probe is compared with that of an

density.

Density is given as grams per

identical reference probe that is shielded

cubic centimeter.

from the corrodent. As the exposed probe ELECTRICAL RESISTANCE METHOD

corrodes, its electrical resistance increase,

Weight-loss measurements indicate the

and this change is related to the extent of

average corrosion rate over a period of

corrosion.

time. Electrical resistance measurement is

sensitive. It is quite similar to the weight-

comparatively a better technique that the

loss coupon method, but enjoys the great

weight loss method.

advantage

In this method, a

This

of

method is

permitting

fast

and

continuous

coupon of material, identical to the alloy

monitoring without removing the coupon. It

whose corrosion rate to be measured is

is also superior to weight-loss method

exposed to the corrodent and periodically

because errors caused by removal of the

withdrawn to measure its loss of weight,

corrosion

which directly relates to corrosion rate. Its

continuous monitoring can indicate the

operation is based on the increase in

effect of process variable on corrosion rate.

electrical

exposed

Electrical resistance probes can serve as an

A metallic

accurate measure of the corrosion rate only

resistance

of

corrosion coupon material.

an

conductor sensing probe, generally a thick

products

is

eliminated

and

when the corrosion is uniform.

wire, strip, or tube of the same material as

*********

Source:

Scraped from “Corrosion of Austenitic Stainless Steels: Mechanisms, Mitigation and Monitoring” edited by H.S.Khatak, Baldev Rai. Chapter ‘Uniform Corrosion of Austenitic Stainless Steel’ by Nisgshen and Mudali.

Pressure Vessel Engineering

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