Thursday, March 5, 2009

Corrosion Coupon

The Weight Loss technique is the best known and simplest of all corrosion monitoring techniques. The method involves exposing a specimen of material (the coupon) to a process environment for a given duration, then removing the specimen for analysis. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss taking place over the period of exposure being expressed as corrosion rate.

The simplicity of the measurement offered by the corrosion coupon is such that the coupon technique forms the baseline method of measurement in many corrosion monitoring programs. The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied which includes, but is not limited to:

• Stress-assisted corrosion
• Bimetallic (galvanic) attack
• Differential aeration
• Heat-affected zones

Advantages of weight loss coupons are that:

• The technique is applicable to all environments - gases, liquids, solids flow.
• Visual inspection can be undertaken.
• Corrosion deposits can be observed and analyzed.
• Weight loss can be readily determined and corrosion rate easily calculated.
• Localized corrosion can be identified and measured.
• Inhibitor performance can be easily assessed.

In a typical monitoring program, coupons are exposed for 90-day duration before being removed for a laboratory analysis. This gives basic corrosion rate measurements at a frequency of four times per year. The weight loss resulting from any single coupon exposure yields the “average” value of corrosion occurring during that exposure. The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss.

Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as electrical resistance, (ER) and linear polarization resistance, (LPR) measurements.

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