Corrosion protection another metal

Dissimilar metals in the same system Because of the specific action of many inhibitors towards particular metals, problems arise in systems containing more than one metal. In the majority of cases these problems can be overcome by the choice of a formulation incorporating inhibitors for the protection of each of the metals involved. With this procedure it is necessary not only to maintain an adequate concentration of each of the inhibitors but also to ensure that they are present in the correct proportion. This is because of two effects firstly, failure to inhibit the corrosion of one metal may intensify the attack on the other metal the best example of this is with aluminium and copper in the same system, and failure to inhibit copper corrosion — usually achieved with sodium mercaptobenzothiazole or benzotriazole—can lead to increased corrosion of the aluminium as a result of deposition of copper from copper ions in solution on to the aluminium surface. Secondly, an inhibitor of the corrosion of one metal may actually intensify the corrosion of another metal. Thus, benzoate is usually used to prevent the corrosion of soldered joints by nitrite inhibitor added to protect cast iron in the same system. A benzoate nitrite ratio of greater than 7 1 is necessary in these cases. 

Sacrificial anode (13.8) A piece of a more reactive metal used to protect another metal from corrosion. The presence of the sacrificial anode assures that the part to be protected will be the cathode and hence will not be oxidized. 

Cathodic protection is a method for protecting metals against corrosion. There are two techniques to achieve this objective. The first approach is to use a sacrificial anode (a less noble metal) and consume it to protect another metal. This technique has been utilized for centuries in marine structures. As pointed out earlier, the galvanic coupling between the two metals results in a current density flowing in the electrolyte as shown in Fig. 6a. In the second approach, known as an impressed current system, the current density is artificially created using an inert electrode. This is depicted in Fig. 6b. Mathematically speaking, the inert electrode can be viewed as a current source where the value of i is assumed to be a known constant at a point. In either case, the intent is to ensure that the... 

Corrosion Inhibition. Another important property of antifreeze solutions is the corrosion protection they provide. Most cooling systems contain varied materials of constmction including multiple metals, elastomeric materials, and rigid polymeric materials. The antifreeze chosen must contain corrosion inhibitors that are compatible with all the materials in a system. Additionally, the fluid and its corrosion inhibitor package must be suitable for the operating temperatures and conditions of the system. 

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

There is a large potential for conducting polymers as corrosion-inhibiting coatings. For instance, the corrosion protection ability of polyaniline is pH-dependent. At lower pH polyaniline-coated steel corrodes about 100 times more slowly than noncoated steel. By comparison, at a pH of about 7 the corrosion protection time is only twice for polyaniline-coated steel. Another area of application involves creation of solid state rechargeable batteries and electrochromic cells. Polyheterocycles have been cycled thousands of times with retention of over 50% of the electrochromic activity for some materials after 10,000 cycles. IR polarizers based on polyaniline have been shown to be as good as metal wire polarizers. 

A number of methods are used to reduce and prevent corrosion. The most common method is to paint iron materials so that the metals are protected from water and oxygen. Alloying iron with other metals is also a common means to reduce corrosion. Stainless steel is an alloy of iron, chromium, nickel, and several other metals. Iron may also be protected by coating it with another metal. Galvanizing refers to applying a coating of zinc to protect the underlying metal. Additionally, because it is a more active metal, zinc oxidizes rather than iron. 

Corrosion of iron (rusting) is an electrochemical process in which iron is oxidized in an anode region of the metal surface and oxygen is reduced in a cathode region. Corrosion can be prevented by covering iron with another metal, such as zinc, in the process called galvanizing, or simply by putting the iron in electrical contact with a second metal that is more easily oxidized, a process called cathodic protection. 

The metals recommended for use with ethanol include carbon steel, stainless steel, and bronze [3.10]. Like methanol, metals such as magnesium, zinc castings, brass, and copper are not recommended. Aluminum can be used if the ethanol is very pure, otherwise it should be nickel-plated or suitably protected from corrosion by another means. The metals compatible with ethanol represent a much wider range than those for methanol and represent most of the metals currently used in fuel systems, so few changes would be anticipated when using ethanol. 

Another industrially important transition metal is titanium, which is used especially to make light but strong industrial parts, such as pipes and airplane propellers. When it is exposed to the air, titanium reacts with oxygen to form a thin layer of titanium dioxide, a compound of titanium and oxygen. The titanium dioxide protects the metal against corrosion, or the gradual breakdown... 

The improvement of existing materials as well as the development of new materials is often based on the use of a chemical reaction in which a solid reacts with another solid, a liquid or a gas to form a solid product (an intermetallic, a silicide, an oxide, a salt, etc) at the interface between initial substances. Therefore, kinetics of solid-state formation of chemical compound layers are of interest not only to chemists (researchers and technologists) but also to metal and solid-state physicists, materials scientists, metallurgists, specialists in the field of corrosion, protective coating, welding, soldering and microelectronics. 

Metals are used for many purposes, but they are often susceptible to corrosion (Chapter 16). Protection against corrosion26 brings huge economic benefits. Often protection is done by electrodeposition of a layer of another metal, more inert (and more expensive) on the substrate27, for example on iron and steels. Because of the importance of efficient protection there has been much laboratory investigation into electrodeposition mechanisms, but there are still empirical factors to be explained satisfactorily. 

Parylene N to smooth surface materials has been reported with the application of plasma depositions [13,14]. It was reported that excellent adhesion of Parylene C coating to a cold-rolled steel surface was achieved using plasma polymer coatings, in turn giving rise to corrosion protection of the metal [15]. Another major deficiency of Parylene C is its poor painting properties when paint is applied on a Parylene C film, due to its extremely hydrophobic surface. Because of this, surface modification of Parylene films is necessary to enhance their adhesion performance with spray primers. 

The free chromium produced in this reaction is laid down in a thin layer on the surface of another metal, such as steel. The chromium protects the steel from corrosion and gives it a bright, shiny surface. Many kitchen appliances are chrome-plated this way. 

Whenever two metals are in electrical contact, a corrosion cell is likely to form. In fact, the metal that is the anode in the cell corrodes faster than it would if it were not connected to another metal. This idea explains the use of sacrificial anodes on ships and pipelines, as shown in Figure 13. As the anode corrodes, it gives electrons to the cathode. The corrosion of the anode slows or stops the corrosion of the important structural metal in a process called cathodic protection. 

The steel hulls of ships are constantly in contact with saltwater, so the prevention of corrosion is vital. Although the hull may be painted, another method is used to minimize corrosion. Blocks of metals, such as magnesium, aluminum, or titanium, that oxidize more easily than iron are placed in contact with the steel hull. These blocks rather than the iron in the hull become the anode of the corrosion cell. As a result, these blocks, called sacrificial anodes, are corroded while the iron in the hull is spared. Of course, the sacrificial anodes must be replaced before they corrode away completely, leaving the ship s hull unprotected. A similar technique is used to protect iron pipes that are run underground. Magnesium bars are attached to the pipe by wires, and these bars corrode instead of the pipe, as shown in Figure 21-15b. 

Iron can be protected from corrosion by applying a coating of another metal or paint to keep out air and water, or by attaching a piece of metal (a sacrificial anode) that is oxidized more readily than iron. 

---