Contacts with other metals

Test Specimens In carrying out plant tests it is necessary to install the test specimens so that they wih not come into contact with other metals and alloys this avoids having their normal behavior disturbed by galvanic effects. It is also desirable to protect the specimens from possible mechanical damage. 

The electrochemical properties of zinc also have a large bearing on its corrosion behaviour. Zinc is negative to Eh /h2 and magnesium and aluminium excepted, to most other metals commonly encountered, including those found in the less pure forms of zinc. This means that when zinc is in contact with these metals sacrificial electrochemical action can take place, with zinc forming the anode. Contact with other metals and impurities can... 

Titanium in contact with other metals In most environments the potentials of passive titanium. Monel and stainless steel, are similar, so that galvanic effects are not likely to occur when these metals are connected. On the other hand, titanium usually functions as an efficient cathode, and thus while contact with dissimilar metals is not likely to lead to any significant attack upon titanium, there may well be adverse galvanic effects upon the other metal. The extent and degree of such galvanic attack will depend upon the relative areas of the titanium and the other metal where the area of the second metal is small in relation to that of titanium severe corrosion of the former will occur, while less corrosion will be evident where the proportions are reversedMetals such as stainless steel, which, like titanium, polarise easily, are much less affected in these circumstances than copper-base alloys and mild steel. 

Nital. A soln of l-5ml of nitric acid (d 1.42 g/cc) in 100ml of 95% ale, used for etching metals. A nital soln contg about 15% by vol of Grasselli reagent, nitric acid and ethanol, used for etching Bi, decomposed vigorously. Explns were also reported when mtal mixts came in contact with other metals. It is believed that the decompn was caused by the reduction of the nitric acid to oxides, which catalyzed the decompn (Ref 1). 

Not stable in uncoated containers. Metals catalyze decomposition of SA. Reacts slowly with copper, brass, and nickel. Contact with other metals may also decompose it. 

Corrosion studies of iron and steel, both alone and in contact with other metals, leading to the concept that iron in contact with nobler metal increased the corrosion rate, while in contact with a base metal resulted in partial or complete protection... 

Foil may corrode in contact with other metals as an electrolytic cell is set up when the product (or condensed moisture) acts as an electrolyte. To minimise corrosion risks, storage in a warm dry place is recommended. 

Table 1.11 Additional Corrosion of Zinc and Zinc Alloys Resulting from Contact with Other Metals or Carbon ... 

In general only poor corrosion behavior is attributed to Mg and its alloys. This is mainly true when Mg is in contact with other metals and alloys in accordance with the electrochemical potential series of metals. Therefore, special care has to be taken to avoid conductive contact between Mg alloys and other metals with different electrochemical potentials. When joining Mg parts with other materials, insulating washers, surface-coated screws or nonconducting films and surface coatings can be applied. When investigating a Mg... 

The generally used expression anodic protection concerns anticorrosion protection methods for some metals and alloys in aggressive chemical environments by anodic polarization with an external dc power source. Maintaining the metal surface in the passive state practically stops corrosion processes. However, it should be mentioned that the anodic passivity phenomenon can occur in the case of contact with other metals characterized by more noble potentials. Cathodic coatings causing anodic passivity are an example. The same role is played by platinum, palladium, or copper additives introduced into steel alloys. This method of protection is sometimes called galvanic anodic protection. 

Of course, those who favored municipal ownership hired their own experts and chemists to challenge the work of Professor Penny. Building a model that was designed to mimic the ultimate Loch Katrine aqueduct, these chemists examined the lead solvency of the water after it passed through the model aqueduct and distribution pipes. This approach was taken because many observers believed that the lead solvency of water was altered by contact with other metals and stones. The upshot of the... 

When in contact with other metals, the ratio of exposed aluminum to that of the more cathodic metal should be kept as high as possible. This reduces the current density on the aluminum. To minimize corrosion, paints and other coatings may be applied to both the aluminum and cathodic metal or... 

When in contact with other metals, pitting corrosion (galvanic corrosion) may occur with UN films [15]. This happens for example, if the invasive medium (for example NaCl solution) penetrates the UN layer due to its porosity and reaches the underlying metals. This corrosion happens also at zero electrode potential. 

When in contact with other metals, titanium alloys are not subject to galvanic corrosion in seawater. However titanium may accelerate attack on active metals such as steel, aluminum, and copper alloys. The extent of galvanic corrosion will depend on many factors such as anode-to-cathode ratio, seawater velocity, and seawater chemistry. The most successful strategies eliminate this galvanic couple by using more resistant, compatible, and passive metals with titanium, alltitanium construction, or dielectric (insulating) joints. 

For a very long time, galvanic corrosion of aluminium in contact with other metals was a major concern to users, to such an extent that it slowed down the development of applications of aluminium whenever the issue of its behaviour in contact with other metals was raised. Nowadays, it is surprising to discover the extensive precautions that were taken in former times with heterogeneous assemblies exposed to ambient atmosphere. 

Most laboratory tests, including electrochemical tests, say nothing about the intensity of possible galvanic corrosion of aluminium in contact with other metals. 

When one of the components of an open or closed circuit happens to corrode, one (or more, in the case of an alloy) of the elements will be dissolved. When thermodynamically possible (potentials of the electrochemical reactions), the ions of this metal may be reduced in contact with other metals present in the circuit, which is why liquids used in closed circuits contain inhibitors to avoid corrosion of all metals and alloys present in the circuit, especially copper and copper alloys, since copper ions Cu can cause severe corrosion of aluminium. This applies especially to the cooling circuits of car engines. 

As in any strongly conductive medium, aluminium is prone to galvanic corrosion in contact with other metals (see Chapter B.3) such as stainless steel. Tests have shown that in boiling nitric acid, the dissolution rate of aluminium is multiplied by five while that of stainless steel is divided by two. 

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