Anodization process

In dipping generally, but particularly with the anode process, it is desirable to use tanks that circulate the coagulant and latex compound, particularly the latter. Use of circulation keeps the Hquid surface clean and free from lumps, scum, or bubbles. Mechanical circulation can cause mbber particle instabihty, however, and eventually coagulate the compound. Therefore, tanks should be designed to minimize friction or shear action, and the compound stabilized to maintain mechanical stabiUty. 

Chemical inhibitors, when added in small amounts, reduce corrosion by affecting cathodic and/or anodic processes. A wide variety of treatments may be used, including soluble hydroxides, chromates, phosphates, silicates, carbonates, zinc salts, molybdates, nitrates, and magnesium salts. The exact amount of inhibitor to be used, once again, depends on system parameters such as temperature, flow, water chemistry, and metal composition. For these reasons, experts in water treatment acknowledge that treatment should be fine tuned for a given system. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

In addition to inspecting for possible contamination, it usually is also of interest to determine whether the chemical etching or anodization process has actually produced the desired oxide. For this purpose, anodization has somewhat of an advantage over etching (FPL, for example) because the thicker oxide developed... 

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

It is apparent from this that since the rates of the cathodic and anodic processes at each electrode are equal, there will be no net transfer of charge in fact, with this particular cell, consisting of two identical electrodes in the same electrolyte solution, a similar situation would prevail even if the electrodes were short-circuited, since there is no tendency for a spontaneous reaction to occur, i.e. the system is at equilibrium and AG = 0. 

If the anodic process is assumed to be solely M rosion of a single metal... 

This area will be passivated by the increase in pH due to the cathodically produced OH ions, and partially cathodically protected by the electrons liberated by the anodic processes within the pit. The tubercle thus results in an occluded cell with the consequent acidification of the anodic sites. Wranglen considers that in view of the fact that crystals of FeClj -4H20 are sometimes observed at the bottom of a pit the solution within the pit is a saturated solution of that salt, and that this will correspond with an equilibrium pH of about 3-5. 

Film-free conditions It has been observed for many metals that the magnitude of / i, (see Section 1.4) increases with temperature and that the activation energy for dissolution is low, suggestive of a diffusion-limited anode process when the migration of corrosion products away from the surface is rate controlling. Some examples of the value of the activation energy for this process are given in Table 2.4. 

Since for the cathodic process the fraction of the energy was 0EF it is reasonable to assume that when the ion travels in the reverse direction during an anodic process the fraction of the energy will be (1 - 0)EF. Thus for the anodic process... 

It can be seen that when tj = 0, / = / o and the rate of the cathodic process equals that of the anodic process, i.e. the reaction is at equilibrium. However, if jj is positive the first term increases exponentially, whilst the second term decreases exponentially, and at overpotentials > -I- 0-052 V the... 

Attention should be drawn to the signs in equations 20.66 and 20.67 and it should be noted that the Tafel slope b is always positive for an anodic process and negative for a cathodic process and that the constant a is of opposite sign to the slope. 

Thus owing to the electrode process the potential will change and will become more negative in a cathodic process and more positive in an anodic process ... 

Of an ion through the double layer towards the electrode and inhibits its transfer in the reverse direction or vice versa f< an, anodic process frequently experirtiental vajups of a aib approximately () 5. Tran assivity active behaviour of a metal atipotehtials, more positive than thbise ieading to passivity. 

Anode processes yield gaseous chlorine, fluorine, carbon chloride or fluoride. In the case of melts containing dissolved tantalum oxide, carbon oxides (mostly carbon dioxide) are formed on the graphite anode [28,37]. 

Oxygen evolution occurs on nickel oxide electrodes throughout charge, on overcharge, and on standby. It is the anodic process in the self-discharge reaction of the positive electrode in nickel-cadmium cells. Early work in the field has been reviewed [9], No significant new work has been reported in recent years. 

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