Weight loss coupon tests

Weight-loss coupon tests are the simplest and most widely used corrosion and inhibitor testing tools employed to determine cumulative metal thinning and... 

Weight loss coupons and other test pieces... 

Probably the weight loss coupon approach is one of the must reliable methods and is widely used. The accuracy of the data is highly dependent on good techniques and on the statistical significance of the tests. The engineering quality data produced require the efforts of many people over a period of several weeks or months, which makes this information quite costly. However, it is the technique of first choice of many processors. 

ASTM D2688 provides a standard test method for measuring the corrosivity of water via weight loss coupons. And the fact is that unless a suitable protocol such as ASTM D2688 is employed for coupon preparation, installation, and subsequent cleaning, variable rates of corrosion are often obtained. Even when protocols are strictly enforced, some variation is usually found, and therefore it is common practice to employ several coupons at the same time, employing a steel or plastic bypass test rack. 

Fig. 15. Corrosion test probes, (a) Electrical resistance (b) linear polarization (c) zero resistance (d) weight loss coupon (e) U-bend specimen (f) tensile specimen and (g) large radius bend specimen.

For atmospheric field and service testing, weight loss coupons have traditionally been used. Modem testing includes in addition electrochemical methods, e.g. with the cell shown in Figure 9.8 [9.13], the quartz crystal microbalance utilizing the frequency response to mass changes [9.14], and fibre optics [9.15]. 

ASTM G 1 Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens Provides methods for cleaning and evaluating weight loss coupons. 

ASTM G 4 Standard Method for Conducting Corrosion Coupon Tests in Plant Equipment Provides guidelines for in-plant testing— weight loss coupons, specimen racks, reporting. 

NACE RP-0775-87 Preparation and Installation of Corrosion Coupons and Interpretation of Test Data in Oilfield Operations Discusses the use of weight loss coupons, interpretation of data, and analysis Principab are appUcable to freshwater systems. 

In addition to classical methods such as weight loss (via coupons), solution analysis for dissolved metal, monitoring of gas evolution (in the case of H2) and change in metal resistance, a select number of electrochemical techniques are widely accepted. This section briefly describes these methods. Weight-loss coupons are still the traditional, accepted baseline for comparisons electrical resistance (ER) and linear-polarization resistance (LPR) are the most widely used electrical and electrochemical techniques. Electrode potential monitoring is sometimes a valuable way of following active/passive transitions. There continues to be an emphasis on the use of non-destructive testing (NDT) techniques (particularly ultrasonic types). 

Several measurements can be made after a coupon-type corrosion sensor has been attached to a cathodically protected pipeline. on potentials measured on the coupon are in principle more accurate than those measured on a buried pipe, if a suitable reference electrode is installed in close proximity to the coupon. The potentials recorded with a coupon sensor may still contain a significant IR drop error, but this error is lower than that of surface on potential measurements. Instant-OFF potentials can be measured conveniently by interrupting the coupon bond wire at a test post. Similarly, longer-term depolarization measurements can be performed on the coupon without depolarizing the entire buried structure. Measurement of current flow to or from the coupon and its direction can also be determined, for example, by using a shunt resistor in the bond wire. Importantly, it is also possible to determine corrosion rates from the coupon. Electrical resistance sensors provide an option for in situ corrosion rate measurements as an alternative to weight loss coupons. 

Laboratory Corrosion Test. The laboratory test procedure for the determination of corrosion rates of coal slurry used in this study was adapted from the method developed by Bomberger (26). The corrosion rates were determined by using ASTM Standard Corrosion Test, also known as the Total Immersion Method (27). The Bomberger technique consists of keeping coal slurry in suspension in a two-liter reaction vessel at a constant temperature. The corrosion rates are determined either by actual weight loss of steel coupons... 

A variety of bench tests exist which attempt to evaluate the oxidative stability and/or the corrosion protection of a particular formulation. These invariably involve subjecting a coupon of one or more metals (chiefly copper and steel) to wet conditions for a fixed period at an elevated temperature. The assessment is carried out on the basis of weight loss and surface discolouration of the coupon. 

Coupon Tests. A weighed metal coupon shaped to conform to the outside surface of a buried pipe is attached by a brazed connecting cable, and both the cable and surface between coupon and pipe are overlaid with coal tar. After exposure to the soil for a period of weeks or months, the weight loss, if any, of the cleaned coupon is a measure of whether cathodic protection of the pipeline is complete. 

Coupon tests are by far the most commonly used test method. The usual method is to mount coupons in the middle of a stream, frequently in a serpentine bypass loop as shown in Fig. 3. Detailed procedures for coupon tests are described in ASTM D 2688, Test Method for Corrosivity of Water in the Absence of Heat Transfer (Weight Loss Methods) (Method A). Tests with very low water flow (suspended coupons in the cooling tower basin, for example) are not recommended unless the objective is to measure corrosion under near stagnant conditions. 

In pulverized coal combustion, the abrasiveness of the particles severely limits the Kfe of the pulverizer grinding elements. The Abrasiveness Index in this application is usually determined by the Yancey-Geer Price apparatus (Babcock and Wilcox, 1992). In this test, four metal test coupons attached to a rotating shaft are rotated at 1440 rpm for a total of 12,000 revolutions in eontact with a sample of 6350 pm x 0 (0.25 in. X 0) coal. The relative Abrasiveness Index is then caleulated from the weight loss of the coupons. Babcock and Wilcox has estimated the wear of full-scale pulverizers on the basis of the Yancey-Geer Price Index. 

Weight-loss cotrports are widely used in REM irrhibitor research tmder variously selected errvirorrmerrlal conditions. Davo et al. carried out a typical coupon test to study the irrhibiting effects of REM salts (CeClj-and LaClj) on Al-Li-Cu (8090) alloy corrosiom The metallurgical phases responsible for the corrosion and irrhibition process were identified with a scarming electron microscope (SEM) coupled to an energy dispersion spectroscopy system (EDS), before and after 1-horrr, 96-horrr arrd 1-month immersion in a 3.56% (w/w) NaCl solution open to air. The compotrrtds formed on the 8090 alloy surface were analyzed by X-ray photoelectron spectroscopy (XPS). 

In the static test, the weight loss of a mild steel coupon after exposure to an inhibited solution is compared to the results obtained with an uninhibited solution under the same conditions. 

The wheel test is a dynamic weight-loss test, wherein a weighed coupon is immersed in the test fluid and rotated on a wheel at fixed rpm and temperature for a set period of time. This coupon is the blank or control. In a system using an identical technique, a known concentration of the inhibitor is used. This test is run simultaneously with the control. At the end of the test period, the amount of weight loss suppression afforded by the inhibitor is determined. The wheel test is a widely accepted laboratory test for two-phase systems such as crude oil. This test is very useful, but not always reliable because of the restricted volume of the solvents and the difficulty in duplicating velocity and stagnation effects in real systems. The test procedure is described in NACE Report 10182. 

Corrosion coupon testing Average corrosion rate over a known exposure period by weight loss or weight gain Most suitable when corrosion is at a steady rate. Indicates corrosion type. Moderately cheap method with corrosion coupons and spools readily made Frequent... 

Most direct measurements of corrosion utilize the weight loss of metal over a period of time on a small sample such as a coupon, wire, or strip. The dimensions of the coupon are important for several reasons. The ratio of surface area to coupon weight should be as high as possible to facilitate detection of small weight losses. This permits the shortest possible exposure period between weightings. Selection of the maximum surface-to-weight ratio, however, may still result in a relatively long test... 

---