Corrosion Coupons
General

The easiest, and longest-used technique for computing corrosion failures in plant and apparatus is weight reduction evaluation. A weighed specimen (coupon) of the metal or amalgam considered is brought into the procedure, and later expelled after a reasonable time interim. The coupon is then sanitized of all corrosion item and is reweighed. The weight reduction is changed over to a corrosion rate (CR) or metal loss (ML).

The method needs no intricate apparatus or techniques, just a properly molded coupon, a bearer for the (coupon holder), and a dependable method of withdrawing corrosion item without disturbance of the metal substrate. Weight reduction estimation is till today the most broadly utilized method for measuring corrosion loss, inspite of being the oldest technique presently being used.

Weight reduction estimation has various attractive characteristics that account for its continuous popularity:

• Easy – No complex instrumentation is needed to acquire an outcome.
• Direct – An immediate estimation is acquired, with no theoretical presumptions or approximations.
• Flexible – It is suitable for all the destructive conditions, and gives data on all types of corrosion.

The technique is generally utilized as a registration standard for different methods of corrosion observing, for example, Linear Polarization and Electrical Resistance. In occasions where slow reaction and calculated data satisfactory, weight reduction observing is the favored method.

Coupon Preparation and Sanitizing

The selection of system for initial planning of the coupon surface, and for sanitizing the coupon after utilization, is crucial in getting helpful information. Both the importance and reproducibility of weight reduction information are very sensitive to the inherent appropriateness of these procedures, and to the care with which they are implemented.

Surface finishing techniques differ over a wide range for particular applications. Blasting with glass bead, sand, or other aggregate can give a satisfactory finish for a few applications. Sanding with grating belts, or surface or twofold disc grinding with rough stones likewise gives a very good surface for assessment.

Sanitization of samples prior to weighing and exposure is vital to withdraw any contaminants that could influence test outcomes.

Coupon Position and Orientation

Regardless of the level of care practiced in the surface preparation of coupons, numerous uncontrollable constituents (e.g. microstructural defects) can diminish the precision of weight reduction judgements.

Coupon positioning must be invariable to make diverse informational sets comparable. Usually, a position parallel to the process stream is prefered since this mostly indicates the genuine condition experienced by the vessel surface. Crystal Industrial’s Corrosion Monitoring Systems’ coupon holders have a programmed stream arrangement characteristic. A marking is made on the top side of all the holders with stream direction for hand-operated alignment.

Placement is another crucial factor in getting important data. For instance, a multi-phase item may produce layered stream, increasing corrosion rates that change with depth in the process flow. Such circumstances can be observed with a coupon holder.

Probably the most widely recognized issue in coupon placing emerges from the fact that a genuine interpretation of the corrosion experienced by the pipe/vessel must be determined when the weight reduction coupon is in the plane of the vessel/pipe surface.