Coupons, deposition

Figure 4.27 Mild steel coupon removed from a rolling mill cooling tank. Note the thick greasy deposits.

Both Linhardt and Dickinson et al." demonstrated that microbi-ologically deposited manganese oxide on stainless and mild steel coupons in fresh water (Fig. 4) caused an increase in Ecorr and increased cathodic current density at potentials above -200 mYscE-" Biomineralization of... 

Stainless steel microelectrodes were prepared by sheathing 100-pm diameter stainless steel wire in glass. Tips were polished on a precision diamond wheel. Electrode response to variation in cathodic depolarizer concentration was confirmed by exposure to solution containing up to 6 mM HjOj. The increase in for the electrode was comparable to the change observed for sample coupons exposed to the same H2O2 concentrations. The electrode was conditioned by exposure to the influent reactor solution for several hours before measuring E, within the biofouling deposits. 

Profiles of DO and HjOj concentrations and microelectrode Ecm were measured within biofouling deposits on coupons. The microelectrodes, mounted in a commercial three-axis micromanipulator, were positioned above a deposit by adjusting the X-Y micromanipulator controls... 

Figure 11. Microelectrode profiles in biofouling deposits on ennobled 316L stainless steel coupons. Horizontal line indicates biofouling/solution interface. (Reprinted from Ref 12 with permission from NACE International.)...

Triple-track resistor and conductor coupons are made by deposition of Ta2N and Ti-Pd-Au metallization, respectively, on the AI2O3 substrate. This test pattern consists of three parallel meandering lines with 3-mil spaces between lines. Each line is approximately 3-mil wide and has 2.86 X ]0 squares, with an overall length of 8.5 in. The number of squares of insulator between adjacent lines is approximately 3.5 X 10". ... 

Fig. 6.5 Typical negative-going LSV recorded on the upper black squares) and bottom (open circles) face of a sterling silver coupon exposed in cultural institutions. The larger peak intensity measuring at — 1.6 on upper face indicates a selective attack of this face that is probably due to the deposition of particulate matter...

However, in the same way that it is unwise for water treaters to rely only on metal coupons when assessing corrosion rates, or only on dip-slides when identifying microbiological infections, information obtained from LSI and SI tools should always be used in conjunction with other sources of related and relevant information. Simple predictive indices should not be used as sole criteria for water management action or change, nor an overreliance placed on them. To obtain high cycles of concentration (COC) yet balance the increased risk of deposition, the use of ion-association predictive modeling is preferred to LSI and SI. 

Fig. 10.2 Various monitoring devices (1 to r) corrosion/fouling/ biofouling mesh coupons, LPRM monitor, corrosion rack, deposit monitor, membrane filter on poured plate, and dip-slide... 

Steel, copper, and most other metal coupons should be cleaned by washing under running water and brushing to remove any loose deposits. If oily, first wash with acetone/isopropanol (IPA). Further clean by immersion in concentrated HCl for 15 seconds. Treat again with HCl and brushing or use of a plastic knife, if necessary, to remove tenacious deposits. Dip in acetone, then IPA, and finally dry in a desiccator prior to weighing. 

Portable deposit/corrosion monitors are typically housed in an enclosure of perhaps 30 in. H x 20 in. W x 15 in. D. Components include inlet flow controller, strainer, adjustable electric heater, (outer) see-through glass housing, (inner) heated specimen tube or block, hot/cold temperature readout, corrosion rack, plus thermal overload, low-flow cut-off, and other safety devices. The specimen tubes or blocks are available in different metals (as are the corrosion coupons) and can usually be replaced in a matter of minutes. Unlike test heat exchangers, the cooling water in this type of monitor flows on the shell side of the specimen tube. 

The deposition of crystalline scales, air-borne contaminants, biofilms, etc. tends to be higher on previously fouled or corroded surfaces than on clean surfaces. Also, deposition and fouling affect both the rates and mechanisms of corrosion on a clean metal surface. It can therefore be useful to obtain subjective information on deposition and fouling tendencies in a cooling system, provided that the methods are simple and produce results quickly. The use of blank coupons inserted in a bypass corrosion rack can often provide this support information. 

For many cooling system operators, the time and expense involved in setting up simulated deposit monitor or heat-exchanger trials is not warranted. However, the simple addition of a blank metal coupon and perhaps... 

Deposition of titania on sulfonic and benzoic acid SAMs on silicon wafers and on PMR-15 and on BMI have been described.17,18 PMR-15 resin coupons were prepared by published procedures.19 Glass BMI prepreg coupons F-650/120 (from Israel Aircraft Industries) were rinsed with acetone and ethanol and dried with filtered N2. Kapton (500HN or VN 125 mm thick, DuPont ) was rinsed with water and ethanol and dried with filtered N2. 

The coupons were dried prior to forming a lap shear joint. About 100 pi of the experimental adhesive was deposited to one coupon and smoothed to cover a 1/2 X 1 area. Another coupon was placed over the area of the initial coupon and a 1 lb weight placed on top. It was cured for 20 to 30 min and the strength of the joint... 

In conducting studies to determine temporal decay rates, sampled areas should be identified so that the same areas are not re-sampled pre-application and post-application. Likewise, if deposition coupons (e.g. a-cellulose filter paper or cotton gauze patches backed with aluminum foil) are placed on the floor during application of the pesticide formulation to estimate initial deposition rates, care should be exercised to avoid sampling in the areas covered by the coupons during subsequent monitoring visits. 

In the first test, conducted in July under hot and relatively dry conditions, the grass was cut to 5 cm and raked prior to application of the chlorpyrifos/ chlorothalonil mixture. Deposition rates were determined by strategic placement of 10-cm X 10-cm cotton gauze coupons backed with aluminum foil. The test plot was allowed to dry for 4 h before sampling. The mean transfer efficiencies for chlorpyrifos were 2.37 ng/m (0.087%) for the PUF roller and 1.05 ng/m (0.039%) for the drag sled. The transfer efficiencies were much higher for... 

General guidance on deposition coupon use is given in Chapter 6 of the Subdivision K, Series 875, Gronp B Guidelines (USEPA, 1998a). 

The mechanism of deposit formation for the three Pennsylvania steam coals and the Decker (Montana) sub-bituminous coal appeared to be as follows (1) iron-rich molten slag drops formed from pyrite-rich mineral particles in the p.c. bonded to the oxidized boiler steel substrate (2) concurrently a layer of fine particles (less than 3 pm) formed a thin layer on the steel coupon surface ... 

A correlation between the corrosivity and the poultice chemistry was made by mounting corrosion test coupons on vehicles in each city and by simulating poultice effects in the laboratory. Results show that corrosion damage increases in a complex manner in areas of heavy road salt use and acid deposition. 

A second finding is that the morphology of the coke deposited on aluminized Incoloy 800 and Incoloy 800 surfaces often was quite different. As depicted in Figures 2B, 2D, 2F, 4D, 4F, 5B> 6B, and 8B> the predominant structure of the coke observed on aluminized surfaces tended to be either a film of tar or a globular coke deposit. Filamentous coke was found on numerous Incoloy 800 samples (Figures 2A, 2C> 2E, 3B> 5C-5F, 7A, and 70 whereas filamentous coke was detected at most in only small amounts on the aluminized surfaces of only three runs (Figures 2F, 7B, and 7D). Third, EDAX analyses indicated appreciable metal, generally mainly iron and nickel, in the coke formed on (or brushed off) the Incoloy 800 surfaces analyses of coke deposited on aluminized Incoloy 800 coupons in the same runs indicated trace amounts of aluminum but no detectable iron, nickel, or chromium. 

Surface heterogeneities described earlier often were important relative to coke formed or deposited on aluminized Incoloy 800 surfaces. Figures SB, 6B, and 10B show globular and cylindrical coke which resulted preferentially in the pitted areas of aluminized surfaces in several runs. Filamentous coke formed at 700 C on an aluminized Incoloy 800 coupon subjected to a 0.05 atm. acetylene feed is depicted in Figure 7D every filament observed was in or near a pitted area but interestingly not every pitted area contained filaments. 

Geometrical Effects For coke deposition on metal samples, the orientation of the coupon face relative to... 

End effects were likely important for Incoloy 800 coupons. Relatively coke—free surface metal) enriched in both chromium and titanium> was visible at the upstream and/or downstream ends of several Incoloy 800 coupons subjected to 0.05 atm. acetylene at 800°C. Numerous small craters> or holes> were observed on the metal. A few isolated filaments protruded from the surface> and sparse amounts of globular coke were also detected at the ends. The bulk of the coke deposit) which consisted of filaments intermixed with larger amounts of globular coke (Figure 8A)> occurred near the midsection of the coupon. A matrix of smooth and rather solid carbon was visible at the metal to coke transition regions the smooth matrix lay under the globular coke. 

To further clarify the role of geometrical effects> the following experiment was performed. Three layered pairs of Incoloy 800 coupons) each pair consisting of one coupon atop a second) were placed in three separate 2.5 cm. lengths of Vycor tubing. One pair was positioned near the furnace midpoint) and the other two pairs were positioned 5 cm. upstream and 3 cm. downstream of the midpoint, respectively. Coke was deposited from 0.05 atm. acetylene at 800°C. Upon completion of the 8 hour run, all three coupon pairs had to be pried apart to separate them. More coke remained on the top coupons than on the bottom coupons (Table 1) for each pair. The top surfaces of all three top coupons exhibited similar types of coke each was covered with a dense layer of spherical-... 

Coupon Type REMOVAL OF COKE % O2 DEPOSITED FROM Feed XH20 ETHYLENE AT 900°C Run Duration, Hours Average Decoking Rate /g coke removed cm2 hr x 101 % burn-off at run end... 

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