Oxide films removal

FIGURE 13.14 Oxide film removal rate versus anionic surfactant concentration. 

Oxide film removal with phosphoric acid was poor, except on the surfaces of coupons 6063/166 and 1100/204. The weights of these coupons after treatment with phosphoric acid were less than their initial weights. 

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Inasmuch as friction conditions determine the flow characteristics of a powder, coarser powder particles of spherical shape flow fastest and powder particles of identical diameter but irregular shape flow more slowly. Finer particles may start to flow, but stop after a short time. Tapping is needed in order to start the flow again. Very fine powders (fine powder particles to coarser ones may increase the apparent density, but usually decreases the flow quality. Metal powders having a thin oxide film may flow well. When the oxide film is removed and the friction between the particles therefore increases, these powders may flow poorly. 

Copper and Copper-Containing Alloys. Either sulfuric or hydrochloric acid may be used effectively to remove the oxide film on copper (qv) or copper-containing alloys. Mixtures of chromic and sulfuric acids not only remove oxides, but also brighten the metal surface. However, health and safety issues related to chromium(VT) make chromic acid less than desirable. 

BM Structure, composition, and properties should be similar and (4) the FM-containing elements should be able to bring about chemical reduction/decomposition or physical removal of BM oxide film. 

The environment plays several roles in corrosion. It acts to complete the electrical circuit, ie, suppHes the ionic conduction path provide reactants for the cathodic process remove soluble reaction products from the metal surface and/or destabili2e or break down protective reaction products such as oxide films that are formed on the metal. Some important environmental factors include the oxygen concentration the pH of the electrolyte the temperature and the concentration of anions. 

Impurities in a corrodent can be good or bad from a corrosion standpoint. An impurity in a stream may act as an inhibitor and actually retard corrosion. However, if this impurity is removed by some process change or improvement, a marked rise in corrosion rates can result. Other impurities, of course, can have very deleterious effec ts on materials. The chloride ion is a good example small amounts of chlorides in a process stream can break down the passive oxide film on stainless steels. The effects of impurities are varied and complex. One must be aware of what they are, how much is present, and where they come from before attempting to recommena a particular material of construction. 

Clean the surface with a wire brush to loosen the oxide film and then wipe it off with a soft cloth. The use of a wire brush serves a dual purpose first, scraping and removing the oxide film, and secondly, providing the surface with a moderate knurling (roughness), which helps to make a better surface-to-surface contact and, in turn, a better joint. 

Tin (powder) [7440-31-5] M 118.7. The powder was added to about twice its weight of 10% aqueous NaOH and shaken vigorously for lOmin. (This removed oxide film and stearic acid or similar material sometimes added for pulverisation.) It was then filtered, washed with water until the washings were no longer alkaline to litmus, rinsed with MeOH and air dried. [Sisido, Takeda and Kinugama J Am Chem Soc 83 538 1961.]... 

So far, we have been talking in our case study about the advantage of an oxide layer in reducing the rate of metal removal by oxidation. Oxide films do, however, have some disadvantages. 

When active, as in a pit or a crevice or when depassivated by mechanical damage of oxide film or chemical removal in nonoxidizing acid. 

Removal of old chromate and fluoride films also removal of corrosion product and oxide films... 

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