Cleaning passive

The decomposition of aqueous hydrogen peroxide is minimized by various purification steps during manufacture, use of clean passive equipment, control of contaminants, and the addition of stabilizers. The decomposition is zero-order with respect to hydrogen peroxide concentration. 

New boilers, heat exchangers, and other equipment must be provided with a PCC program to remove oil, grease, mill scale, pipe-threading compounds, drawing compounds, and other detritus that otherwise prevent the formation of clean, passivated metal surfaces and encourage... 

Most cleaning/passivation formulations tend to be fairly simple and low cost, especially when large cooling systems are involved, although more complex, multifunctional formulations do exist. The products are added to the system water at 0.5 to 2.5% v/v, depending on system size, with the amount of phosphate initially present usually held to around 500 to 1000 ppm PO4. 

Many closed-loop hot/cold/chilled water systems are not treated, perhaps due to the mistaken notion that treatment is not required. Only when severe corrosion and fouling/blockages problems occur is something done, by which time it is normally too late to fully recover the situation. Closed-loop systems should always be cleaned, passivated, and treated. Treatment is most usually applied by means of a dosing pot (bypass feeder) installed on a bypass line. Some operators also install sidestream filters to remove suspended iron oxide. This is most probably not necessary if the... 

In an effort to keep the copper surfaces clean, passivation strategies have been developed to inhibit the reformation of oxides on the surface in an effort to minimize recontamination. The performance of the cleaning process and subsequent passivation were investigated by Hoppe, et al. ... 

The ERICE applications currently under development at ENEA refer to the management of power plant standard chemical treatments (acid cleaning, passivation, additive treatments, etc.), and to the plant operation. Since these applications demand the specific knowledge for each plant to be monitored, they will be developed in close cooperation with firms active in the specific sectors. 

Liquid fluorine can be handled in clean passivated equipment made of Monel, aluminum, types 304 and 321 stainless steels, copper, and brass. Equipment that contains fluorine must not be bent, flexed, or struck as the protective fluoride film may be removed permitting the fluorine to react violently with the fresh metal surface. 

Corrosion is often local, with a few centimetres of corrosion and then up to a metre of clean passive bar, particularly for chloride induced corrosion. This indicates the separation of the anodic reaction (2.1) and the cathodic reaction (2.2) to form a macrocell . Chloride induced corrosion gives rise particularly well defined macrocells. This is partly due to the mechanism of chloride attack, with pit formation and with small concentrated anodes being fed by large cathodes. It is also because chloride attack is usually associated with high levels of moisture giving low electrical resistance in the concrete and easy transport of ions so the anodes and cathodes can separate easily. 

Special commissioning requirements (such as chemical cleaning, passivating, or testing)... 

Adherends Surface Preparation 303 stainless steel Sandblasted Cleaned Passivated at RT using sodium dichromate/nitric acid 6061 T6 aluminum Sandblasted Cleaned... 

Equipment should be carefiiUy and completely degreased and passivated with low concentrations of fluorine or the gaseous halogen fluoride before use. Special care should be taken that valves are completely disassembled and each part carefiiUy cleaned. 

Monel and nickel are the preferred materials of constmction for cylinders and deHvery systems however, copper, brass, steel, and stainless steel can be used at room temperature, providing that these metals are cleaned, dried, and passivated with a fluoride film prior to use. Studies have shown that fluorine passivation of stainless steel and subsequent formation of an iron fluoride layer prior to WF exposure prevents reaction between the WF and the stainless steel surface (23). 

Metal Cleaning. About 204 thousand metric tons of HCl (100% basis) was consumed in 1993 for steel pickling, wherein the hydrochloric acid readily dissolves all of the various oxides present in the scale formed during the hot rolling process. Using suitable inhibitors such as alkyl pyridines, HCl reacts very slowly with the base metal rendering the surface so clean that it must be passivated with a mild alkaline rinse. 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

Zirconium is a highly active metal which, like aluminum, seems quite passive because of its stable, cohesive, protective oxide film which is always present in air or water. Massive zirconium does not bum in air, but oxidizes rapidly above 600°C in air. Clean zirconium plate ignites spontaneously in oxygen of ca 2 MPa (300 psi) the autoignition pressure drops as the metal thickness decreases. Zirconium powder ignites quite easily. Powder (<44 fim or—325 mesh) prepared in an inert atmosphere by the hydride—dehydride process ignites spontaneously upon contact with air unless its surface has been conditioned, ie, preoxidized by slow addition of air to the inert atmosphere. Heated zirconium is readily oxidized by carbon dioxide, sulfur dioxide, or water vapor. 

Metal Cleaning. Citric acid, partially neutralized to - pH 3.5 with ammonia or triethanolamine, is used to clean metal oxides from the water side of steam boilers and nuclear reactors with a two-step single fill operation (104—122). The resulting surface is clean and passivated. This process has a low corrosion rate and is used for both pre-operational mill scale removal and operational cleaning to restore heat-transfer efficiency. 

In contrast to external protection, the anodes in internal protection are usually more heavily covered with corrosion products and oil residues because the electrolyte is stagnant and contaminated. The impression can be given that the anodes are no longer functional. Usually the surface films are porous and spongy and can be removed easily. This is achieved by spraying during tank cleaning. In their unaltered state they have in practice little effect on the current output in ballast seawater. In water low in salt, the anodes can passivate and are then inactive. 

Check continuity zero and adjust Calibrate as required Chemical Cleaning Activation Passivation Rinse... 

Clean-out of the lubrication system, including chemical cleaning and passivation, where required. 

Selection of materials of construction compatible with the chemical(s) in use, properly cleaned and passivated... 

Piping systems should be designed for an economic flow velocity. For relatively clean fluids, a recommended velocity range where minimum corrosion can be expected is 2 to 10 fps. If piping bores exist, maximum fluid velocities may have a mean velocity of 3 fps for a 3/8-in. bore to 10 fps for an 8-in.-diameter bore. Higher flow velocities are not uncommon in situations that require uniform, constant oxygen supply to form protective films on active/passive metals. 

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