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By A. S. Krisher
Corrosion Testing
Why?
Corrosion tests are conducted for a number of reasons, some of which are:
The discussion which follows relates primarily to reason No.4, although the same
principles apply in tests conducted for other reasons.
General Requirements for Coupon Tests
There are a number of "good practice" requirements which apply to all coupon testing.
Type of Tests
Data of value in estimating the probable service life of a piece of process equipment can be generated in a number of ways.
Surface Condition of Coupons
Surface condition for coupons is a subject of substantial debate. A typical vessel as installed in the plant will have large surface areas in original mill condition, smaller areas of weld heat-affected zone, and areas that have been ground during the fabrication process.
It is possible to replicate all of these conditions on a coupon. However, in the
interest of simplicity and consistency, it is fairly common to machine the coupon surface
flat, leaving a standard ground surface as defined by the size grading of the grinding
media. An example would be the "120 grit finish" achieved by grinding with a 120
grit belt. This leaves a flat surface with clearly detectable scratches, all in a
consistent direction. Any deviation from this standard initial surface is attributable to
the exposure in the environment. Any effect on corrosion due to the initial finish, or the
as welded heat-affected zone finish, or the ground surface will be very temporary in a
corrosive system.
It is also worth noting that if the surface condition (i.e., the mill finish) does
provide an improved corrosion resistance over the metal without this surface condition,
such a situation will have questionable reliability in an operating system. When this
surface condition is damaged mechanically or chemically, the corrosion resistance will
revert to that of the parent metal without the special surface treatment.
Coupon Mounting
Coupons should be mounted in such a way that they are securely held and are
electrically isolated from contact with all other metals (except when the purpose of the
test is to study galvanic corrosion). Mounting materials (brackets, bolts, etc.) and
insulating materials should be selected to be fully resistant to the environment. Failure
of any of these components will lead to loss of data or loss of electrical isolation.
Time of Test
In general, coupon tests should be run for a minimum of 1 week. In many cases, it will
be worthwhile and desirable to evaluate the effect of time of exposure which can be done
by means of a controlled interval test.
Economics of Corrosion Testing
Corrosion testing is not cheap. More specifically, materials for a field rack with ten
coupons will cost about $150 with 316 hardware or $250 with Hastelloy C-276 hardware. If a
field test program required ten such racks, the total cost would be $1500 to $2500 plus
the direct costs (rack assembly, rack installation and removal, record keeping,
evaluation, reporting, etc.)
These costs should be evaluated in terms of the benefits derived from the information
generated by the tests.
In today's process industry, direct maintenance costs associated with a premature corrosion failure usually run to (at least) tens of thousands of dollars, and frequently into the hundreds of thousands. The business losses associated with such failures can easily be ten times the direct maintenance costs.
Considered in this fashion, it seems evident that the expenses of corrosion coupon
testing can be easily justified.
Coupon Evaluation after Exposure
At the end of the test, observations of the coupon before cleaning should be recorded
(photographically if appropriate). Samples are cleaned by various means (detailed in
appropriate specifications) to remove all deposits and corrosion products from the
unreacted metal. After cleaning, the coupon is weighed again and the corrosion rate is
calculated from the weight loss.
The constant can be varied to calculate the corrosion rate in various units:
| Desired Corrosion Rate Unit (CR) | Area Unit (A) | K-Factor |
| mils/year (mpy) | in2 | 5.34 x 105 |
| mils/year (mpy) | cm2 | 3.45 x 106 |
| millimeters/year (mmy) | cm2 | 8.76 x 104 |
| Desired Metal Loss Unit (ML) | Area Unit (A) | K-Factor |
| mils | in2 | 61.02 |
| mils | cm2 | 393.7 |
| millimeters | cm2 | 10.0 |
Note that this calculation yields an average rate, assuming perfectly even metal loss from
all surfaces. Examine coupons under low power magnification and record evidence of
localized attack. End grain attack, localized weld attack, intergranular corrosion,
accelerated attack in stressed area (at the stenciled numbers), and localized attack
associated with the mounting hardware should be noted if present. The depth of penetration
of localized attack should be determined by means of microscopic examination or
metallographic examination.
Advantages of Coupon Testing
Coupon tests are low in cost, simple to conduct, and allow the simultaneous evaluation
of numerous materials and variations of a single material. Alloy chemistry variations and
metallurgical variations (ie., the effect of heat treatment, microstructure, welding and
stress) can be considered. Coupon tests are easily adapted to evaluate specific types of
corrosion, such as crevice corrosion and galvanic corrosion.
Summary
Coupon testing remains a powerful tool in the corrosion engineer's tool kit.
Intelligent and systematic use of this tool provides data which allows a knowledgeable and
experienced engineer to make reliable predictions of field performance.
To Dig Deeper
There is a large body of literature dealing with corrosion coupon testing. The references listed below will serve as a useful introduction.
Contact Metal Samples Corrosion Monitoring Systems for more information.
Main Office:
Phone: (256) 358-4202
Fax: (256) 358-4515
E-mail: msc@alspi.com
