Coupon exposures

Figure 38 Comparison of data from SL-EPR, acid ferric sulfate, and oxalic acid etch test for seven separate heats of Type 304 and 304L stainless steel. Note that for low levels of sensitization, the SL-EPR can quantitatively distinguish among degrees of sensitization. At higher levels, the coupon exposure tests are more discriminating. (From Ref. 32.)...

There is an abundance of compatibility information provided by resin manufacturers. The information can be of two kinds. One is actual coupon exposure data with no judgement as to the suitability, and other is a list of chemicals against which a rating system (A, B, C, etc.) is given. Often the maximum operating temperature is also given as additional useful information. 

Based on preliminary EDS results indicating low total suspended solids in ICB effluent, the sludge clarifier/ thickener might be eliminated. Off-site disposal of concentrated brine rather than dried salts would eliminate the need for the crystallizer and solids filtration steps. Off-site brine disposal would also be consistent with current operations at Tooele, Utah, and planned operations at Newport, Indiana. The basis for the currently designated materials of construction for the evaporator and crystallizer is unclear. If on-site crystallization is included in the final process design, a more extensive evaluation of the materials of construction will be necessary, including stressed-materials coupon-exposure tests. The dried solids from the crystallization step might be... 

Because detailed information on coupon testing is available elsewhere, this section will focus on the aspects of such testing that are specific to the evaluation of pitting corrosion. When performing coupon exposures, it is important to include alloys whose corrosion behavior in the environment of interest is well characterized and reproducible to serve as an internal standard. This protocol is of particular importance when localized corrosion is studied. Because pitting has a stochastic nature, the use of these controls" increases the confidence with which final material selection choices are made. If the control materials behave as expected, it is probable that the results for the other materials reflect typical behavior for the environment of interest. 

Corrosive gas testing became commonplace in the mid-1970s as a result of the need of the electronics industry to reduce the cost of materials used as electrical contacts on connectors and as conducting pathways on various kinds of insulating substrates. Studies at IBM [75-78] and Battelle [79] played a key role in the development of this technology. Studies at both institutions compared laboratory data to field data and achieved results in the laboratory that accurately reproduced field results. Field exposures of 20 years can be simulated in 5-30 days in the laboratory. Based on metal coupon exposures and simultaneous air px>llutant measurements from over a hundred locations around the world, a four-level environmental classiflcation scheme for environmental severity, which is now in common use around the world, was developed by the industry-sponsored Battelle group. 

Field corrosion testing consists of coupon testing and probe testing. Coupon exposure times typically rtinge from one to three months. Probes are exposed for as long as a specific condition is under investigation, sometimes for the life of the process. 

Bleach plant corrosion testing has included corrosion coupon testing, electrochemical testing, electrochemical protection testing, and nonmetallic materials evaluation. Coupon exposure tests have been used to rank candidate materials for these applications. Nonmetallic materials including thermoplastics, thermosets, brick, and tile are widely used in the bleach plant. 

The simplest form of direct atmospheric corrosion measurement is by coupon exposures. Subsequent to their exposure, coupons can be subjected to weight-loss measurements, pit density and depth measurements and to other types of examination. The main drawback associated with conventional coupon measurements is that extremely long exposure times are usually required to obtain meaningful data, even on a relative scale. It is not uncommon for such programs to run for 20 years or longer. 

Kinetic information and an overall picture of the material s corrosion behavior can be obtained relatively quickly (compared to, say, coupon exposures). 

When either of these techniques is used, however, inhibitor treatment of the complete stream is necessary for the relatively long times needed for coupon exposure. To minimize the test times, electrical resistance probes, polarization resistance electrodes, or iron counts can be used, when applicable, reducing test times to days instead of weeks. 

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