Corrosion products removal

Figure 4.22 As in Fig. 4.21 hut with deposits and corrosion products removed to reveal numerous depressions. (Magnification 7.5x.)... 

Electrolyte-sulphuric acid (5% wt.%) plus an inhibitor (0-5kgm ) such as diorthotolyl thiourea, quinoline ethiodide or /3-naphthol quinoline. The temperature should be 75°C, the cathode current density 2000 Am and the time of cathodic polarisation 3 min. The anode should be carbon or lead. If lead anodes are used, lead may deposit on the specimens and cause an error in the weight loss. If the specimen is resistant to nitric acid the lead may be removed by a flash dip in 1 1 nitric acid. Except for this possible source of error, lead is preferred as an anode, as it gives more efficient corrosion product removal. 

If it is necessary to minimize the uncertainty of the corrosivity result, the surface area determination and surface roughness specification should be improved as well as mass loss determination, which is based on a rather complicated procedure of corrosion product removal. 

In cases where runoff or spalling losses are significant, must be estimated from an analysis of the corrosion product retained on the corroding metal. This was done by wet chemical analysis of the stripping solutions that contain the corrosion product removed from exposed samples. Again, if the corrosion product is thermodynamically stable for a wide range of environmental... 

In order to determine the value of e the amount of corrosion product removed from the surface, it is necessary to integrate Equation 10.44. 

Third, a hybrid procedure is being used frequently where conditioned fluids are circulated over the test specimen from a reservoir [20]. If the reservoir is large enough, changes in fluid composition can he minimized. A particularly ingenious version of this hybrid test method has been developed in Norway [23], where the circulating fluids are continuously reconditioned by automatic pH maintenance and corrosion product removal. The distinction of these three approaches must always be kept in mind when discussing corrosion and inhibition tests. 

The weight gains ( 6 and 7) could be experimental errors caused by some reaction products deposited from the corrosion product removal solution or a passive film/layer formed in the coolants. 

The possible remedial and preventive actions are hot soaks and drains during cooldown to help remove soluble deposited material, chemical cleaning to remove corrosion products and reduce the pressure drop (see Metal surface treatments), and reduced corrosion product transport into OTSG using amines other than ammonia in feedwater (14). 

Precipitate formation can occur upon contact of iajection water ions and counterions ia formation fluids. Soflds initially preseat ia the iajectioa fluid, bacterial corrosioa products, and corrosion products from metal surfaces ia the iajectioa system can all reduce near-weUbore permeability. Injectivity may also be reduced by bacterial slime that can grow on polymer deposits left ia the wellbore and adjacent rock. Strong oxidising agents such as hydrogen peroxide, sodium perborate, and occasionally sodium hypochlorite can be used to remove these bacterial deposits (16—18). 

Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

Ideally, a system for recycling spent antifreeze consists first of the removal of the deleterious contaminants such as the corrosion products, corrosive ions, degradation products, and remaining inhibitors. Then the clean fluid could be reinhibited to a known concentration of both inhibitors and glycol. 

Impingement Corrosion This phenomenon is sometimes referred to as erosion-corrosion or velocity-accelerated corrosion. It occurs when damage is accelerated by the mechanical removal of corrosion products (such as oxides) which would otherwise tend to stifle the corrosion reac tion. 

Whenever corrosion resistance results from the accumulation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be either to prevent their normal formation or to remove them after they have been formed. Either effect allows corrosion to proceed unhindered. This occurs frequently in smaU-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of oends in pipe hnes, and on propellers, agitators, and cen-trifiig pumps. Similar effects are associated with cavitation and impingement corrosion. 

Duration of Test Although the duration of any test will be determined by the nature and purpose of the test, an excellent procedure for evaluating the effect or time on corrosion of the metal and also on the corrosiveness of the environment in laboratory tests has been presented by Wachter and Treseder [Chem. Eng. Pi og., 315-326 (June 1947)]. This technique is called the planned-interval test. Other procedures that require the removal of sohd corrosion products between exposure periods will not measure accurately the normal changes of corrosion with time. 

Mechanical cleaning includes scrubbing, scraping, brushing, mechanical shocking, and ultrasonic procedures. Scrubbing with a bristle brush and a mild abrasive is the most widely used of these methods the others are used principally as supplements to remove heavily encrusted corrosion products before scrubbing. Care should be used to avoid the removal of sound metal. 

Electrolytic cleaning should be preceded by scrubbing to remove loosely adhering corrosion products. One method of electrolytic cleaning that has been found to be useful for many metals and alloys is as follows ... 

Figure 4.15 As in Fig. 4.14 but with silt removed to reveal bluish-white copper carbonate corrosion products. 

Internal surfaces exhibited many rounded, mutually intersecting pits partially buried beneath silt, iron oxide, and sand deposits. Orange and brown corrosion products and deposits overlaid all. Sulfides were present in the deposits and corrosion products. The material was easily removed when acid was applied (Figs. 4.21 and 4.22). 

X-ray analysis of corrosion products and deposits removed from internal surfaces showed 68% iron, 12% phosphorus, 8% silicon, 3% sulfur, and 2% each of zinc, sodium, chromium, and calcium other materials made up the remainder of deposits and corrosion products. 

Corrosion products and deposits. All sulfate reducers produce metal sulfides as corrosion products. Sulfide usually lines pits or is entrapped in material just above the pit surface. When freshly corroded surfaces are exposed to hydrochloric acid, the rotten-egg odor of hydrogen sulfide is easily detected. Rapid, spontaneous decomposition of metal sulfides occurs after sample removal, as water vapor in the air adsorbs onto metal surfaces and reacts with the metal sulfide. The metal sulfides are slowly converted to hydrogen sulfide gas, eventually removing all traces of sulfide (Fig. 6.11). Therefore, only freshly corroded surfaces contain appreciable sulfide. More sensitive spot tests using sodium azide are often successful at detecting metal sulfides at very low concentrations on surfaces. 

Erosion-corrosion can be defined as the accelerated degradation of a material resulting from the joint action of erosion and corrosion when the material is exposed to a rapidly moving fluid. Metal can be removed as solid particles of corrosion product or, in the case of severe erosion-corrosion, as dissolved ions. 

Affected areas are essentially free of deposits and corrosion products, although these may be found nearby (Fig. 11.4). Affected areas may be covered with deposits and corrosion products if erosion-corrosion occurs intermittently, and the component is removed following a period in which erosion-corrosion was inactive. 

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