Titanium anodic passivation

Anodic passivation also allows titanium to be employed as a Jig for aluminium anodising baths ", because the protective anodic film formed on titanium allows passage of electronic current to the metal contact while virtually suppressing flow of ionic current through the anodically-formed surface film. This aspect is discussed in more detail in relation to special applications. 

Because of its good performance in mineral acids, there is little need or incentive to invoke anodic passivation techniques for zirconium. The metal can be anodised in sulphuric acid, but, again in contrast to the behaviour of titanium, it does not form a stable anodic film in chloride solutions, and even in neutral sodium chloride, zirconium rapidly corrodes if an anodic potential of 2 V is applied. 

The Operational Characterisics of Platinised-Titanium Anodes Platinised-titanium anodes have the disadvantage that the protective passive him formed when titanium is made anodic in certain solutions can breakdown. This could result in rapid pitting of the titanium substrate, leading ultimately to anode failure. The potential at which breakdown of titanium occurs is dependent upon the solution composition, as is evident from Table 10.16. 

Because of its good passivity, titanium is also suited for anodic protection (e.g., in mineral and organic acids, tantalum cathodes are generally used) (Cotton 1960). 

The beneficial effect of cathodic areas can be explained [7] as one of anodic passivation or anodic protection of aluminum in the same manner as alloying additions or coupling of platinum or palladium to stainless steels or to titanium passivate these metals in acids (Section 6.4). 

The function of alloyed copper is the same as that of alloyed palladium in titanium mentioned in Section 6.4, namely, to accelerate the cathodic reaction or O2 reduction) to the point where the anodic current density reaches or exceeds the critical value for anodic passivation. 

The corrosion rate of anodized titanium (solution 60 ml ethanol, 35 ml water, 10 ml lactic acid, 5 ml phosphoric acid, 5 mg citric acid and 5 mg oxalic acid 45V, 45s) is much lower than that of passivated titanium (40%... 

Solid p2ttticle erosion in salt water was found to increase anodic passivation in titanium. The rate of erosion for pure titanium wtis compared with the same for titanium alloys. It was found that titanium tJloys like Ti-6Al-4V showed significant increased resist2uice to erosion-corrosion. The addition of 0.1 % Ru to Ti-6Al-4V further increased the erosion-corrosion resistance of the alloy. 

Platinised titanium anodes (titanium carrying a thin surface film of platinum, of the order of 0-0025 mm thick) have proved successful in cathodic-protection systems employing impressed-current techniques, as electrodes for electrodialysis of brackish water, and in many applications where established anode materials suffer significant corrosion. Platinum-coated titanium anodes can operate without breakdown at very high current densities, of the order of 5 0(X)A/m, in sea water, as although the very thin platinum coating may be porous the underlying titanium exposed at the pores will become anodically passivated... 

Anodic protection is particularly suitable for stainless steels in acids. Protection potential ranges are given in Section 2.4. Besides sulfuric acid, other media such as phosphoric acid can be considered [13,21-24]. These materials are usually stable-passive in nitric acid. On the other hand, they are not passivatable in hydrochloric acid. Titanium is also a suitable material for anodic protection due to its good passivatability. 

The anodic behaviour of platinum and certain of its alloys is of considerable technical importance, since they can be employed under a wide range of conditions without appreciable corrosion, and often in circumstances where no other metal can be used. Their use industrially has recently been extended by applying them as thin coatings to a substrate of a passive metal such as tantalum or, more commonly nowadays, titanium, to reduce the cost. Platinised titanium anodes are discussed in detail in Section 11.3. 

The addition of a small percentage of a noble metal to a base metal such as stainless steel or titanium can provide sites of low overvoltage for the cathodic reduction of dissolved oxygen or hydrogen ions. This permits larger currents and hence more positive potentials to be obtained at the anodic region, and promotes passivation under some circumstances . This effect has been demonstrated for stainless steels but has not been adopted in practice, since under other conditions the noble metal addition accelerates corrosion . 

A number of bi-electrodes have been studied for application as insoluble anodes in electroplating platinised titanium, Ti-Pt, Ti-Cu and Ti-Ag. Anodic polarisation measurements in various copper, nickel, chromium and tin plating solutions together with passivation current densities are used to discuss performance and suitability. 

Table 10.9 lists some common zinc anode alloys. In three cases aluminium is added to improve the uniformity of dissolution and thereby reduce the risk of mechanical detachment of undissolved anode material . Cadmium is added to encourage the formation of a soft corrosion product that readily crumbles and falls away so that it cannot accumulate to hinder dissolution. The Military Specification material was developed to avoid the alloy passivating as a result of the presence of iron . It later became apparent that this material suffered intergranular decohesion at elevated temperatures (>50°C) with the result that the material failed by fragmentation". The material specified by Det Norske Veritas was developed to overcome the problem the aluminium level was reduced under the mistaken impression that it produced the problem. It has since been shown that decohesion is due to a hydrogen embrittlement mechanism and that it can be overcome by the addition of small concentrations of titanium". It is not clear whether... 

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