Corrosion monitoring and control

Corrosion Theory 1259. Forms of Corrosion Attack 1268. Factors Influencing Corrosion Rate 1292. Corrodents in Drilling Fluids 1300. Corrosion Monitoring and Equipment Inspections 1312. Corrosion Control 1323. Recommended Practices 1340. 

Water treatment monitoring and control is often a knife-edge operation and must be tailored to the overall operation of the boiler because waterside and gas-side problems usually are interlinked. Consequently (and as with other types of WT boiler), not only should the utility boiler FW be essentially free of dissolved oxygen to prevent waterside pitting corrosion of the economizer and other boiler components, but also the temperature must be high enough to prevent dewpoint condensation and subsequent acid attack on the gas side of the economizer tubes. 

Within these tortuous systems there exists considerable opportunity for process contamination, corrosion, and equipment malfunction to occur, with cause-and-effect problems creating further impact downstream and placing additional demands on monitoring and control efforts. 

Where chlorine dioxide is used in large process cooling systems, it is not uncommon for significant levels of sulfides to be present in the cooling water system. A benefit of chlorine dioxide biocide is its additional ability to readily oxidize this sulfide however, good monitoring and control is necessary, as high initial sulfide levels can result in mineral acid formation and the potential for corrosion to occur ... 

Gaseous ozone is toxic it can pose a safety hazard and can also be corrosive. Thus good quality generators and installations are required, as well as adequate monitoring and control systems. (Although, as it is site-generated, production ceases immediately when the equipment is turned off and the gas will decompose naturally). 

The renewable energy processes will be monitored and controlled by small, fast, and economical detectors, and fiber optics will play a major role in their designs. The operation and applications of fiber-optic probes have already been discussed in connection with Figure 3.2, so only a brief summary is provided here. Fiber-optic probes can be installed in situ, whereas their readout instruments can be several hundred meters from the probe. The probe can be located in toxic, corrosive, radioactive, explosive, high- or low-temperature/ pressure, and noisy environments. Because the measurement signal is optical, the cables are immune to microwave or electromagnetic interference. 

The task of purging pipelines for maintenance is almost second-nature to well-experienced operators in this unit. Typically, pipeline clearing is routine and is uneventful. This time, however, the utility dry air system was also being utilized as a source for instrument air in the operating area. Hence, this corrosive material was able to backflow throughout the instrument air system into monitoring and control systems. The backflow created expensive instrument damages. 

If materials selection depends on corrosion control by process-related measures (such as chemical treatment), these should be indicated on the MSD. Indicate the intended injection points and the type of chemical to be injected. Examples include corrosion inhibitors, scale inhibitors, biocides, pH control chemicals, wash water, etc. Also indicate the location of proposed corrosion monitoring and sampling sites. If anodic or cathodic protection is to be part of the corrosion control design, the MSD or its Notes section should indicate the piping and/or equipment to be protected. 

The pH process monitoring and control equipment is standard industrial grade instrumentation and is available from numerous vendors. The pH measurement is commonly made in the froth flotation cells, and caustic soda or lime is added to the tailings output of the froth cells. This measurement is primarily used to control the pH of the plant process water with the set point selected to yield optimum performance from the flocculants added to the static thickener. Control of the process water acidity is also required to minimize the corrosion of process equipment. 

The pump forces the mobile phase through the column at high pressure and is monitored and controlled by the pressure regulator. Different pump types are available binary and quaternary pumps are the most frequently employed. A binary pump can drive two mobile phases at the same time, whereas a quaternary pump can drive four different mobile phases at the same time, if required. The pump must be able to generate high pressures, deliver flow rates from 0.1 to 10 mL min h have reproducibility of 0.5% (or better), and be resistant to corrosion by a variety of solvents. 

S. Maxwell, C. Devine, F. Rooney, I. Spark. Monitoring and control of bacterial biofihns in oilfield water handling systems. Paper No. 04752, Corrosion 2004, NACE International, Honston, TX, 2004. 

Figure 9.14 Cathodic protection with an impressed current, (a) Leadjsilver anode for the protection of a ship s hull. Note that the anode is insulated from the hull and surrounded by a non-conducting shield to improve the potential distribution over the hull The anodes will be placed at intervals around the hull (b) Anode cans for the protection of an underground pipeline (some distance away). The anode cans contain a Fe/Si rod surrounded by coke breeze (to increase anode area), (c) Monitor and controlled power supply for a ship s corrosion protection system. Photographs supplied by Corrintec UK Ltd.

Morgan, J. (1975). Monitoring and control of cathodic protection of offshore structures, Corrosion 75, Toronto, NACE, Paper 20, 8 pp. 

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