Iron continued corrosion rates

If the amount of metal removal by erosion is significant the surface will probably be continually active. Metal loss will be the additive effect of erosion and active corrosion. Sometimes the erosion rate is higher than that of active corrosion. The material selection judgment can then disregard coirosion and proceed on the basis of erosion resistance provided the corrosion rates of aetive surfaces of the alloys considered are not much different. As an example of magnitudes, a good high-chromium iron may lose metal from erosion only a tenth as fast as do the usual stainless steels. 

A comprehensive table of corrosion rates in sea water has been compiled by LaQue . This appears to show no obvious dependence of corrosion rates on the geographical location of the testing site, and few of the rates depart widely from an average of 0-11 mm/y. It is suggested that a figure of 0-13 mm/y may be taken as a reasonable estimate of the expected rate of corrosion of steel or iron continuously immersed in sea water under natural conditions, in any part of the world. 

In sodium chloride solutions, on the other hand, the conductivity is greater hence, additional anodes and cathodes can operate much further removed one from the other. At such cathodes, NaOH does not react immediately with FeCl2 formed at anodes instead, these substances diffuse into the solution and react to form Fe(OH)2 away from the metal surface. Any Fe(OH)2 so formed does not provide a protective barrier layer on the metal surface. Hence, iron corrodes more rapidly in dilute sodium chloride solution because more dissolved oxygen can reach cathodic areas. Above 3% NaCl, the continuing decreased solubility of oxygen becomes more important than any change in the diffusion-barrier layer hence, the corrosion rate decreases. 

Reduction of passivator continues at a low rate after passivity is achieved, equivalent in the absence of dissolved oxygen to the value of ipasave [<0.3 pA/cm (<3mA/m )] based on observed corrosion rates of iron in chromate solutions. Iron oxide and chromate reduction products slowly accumulate. The rate of reduction increases with factors that increase /passive, such as higher activity, higher temperatures, and the presence of CF. It is found, in practice, that less chromate is consumed as exposure time continues, consistent with /passive that also decreases with time. 

Since the pH is probably the most important of the controlling test parameters, particularly in CO2 corrosion, it can be held constant by continuous pH controlled acid injection as the corrosion process progresses [9]. This prevents the build-up of iron carbonate scale. Indeed, in the constant pH test, using a brine saturated with CO2 at one bar, the blank corrosion rate was found to be constant (or slightly increasing) at about 220 mpy for at least 48 h, whereas in the simple constant inventory test without pH compensation the corrosion rate would decrease from an initial 220 mpy to 20 mpy after 24 h due to iron carbonate "passivation," with a concomitant pH increase of from 4 to about 6. 

Bertocci et al. (1997 a, b) continued and extended the theoretical analysis, showing the differences among the various measurement methods, as well as the specific problems arising from the different circuit schemes. Figure 7-22 shows the spectral noise resistance / sn comparison to Z obtained by impedance measurements of iron in 1 M Na2S04 solution at pH 4 (Bertocci et al., 1997b). This system is quite stable, allowing measurements at very low frequencies. The corrosion at this pH is uniform and the corrosion rate is low. Hydro-... 

Hie specific acid anion can also affect the corrosion rate. Some acids, such as H2S, react with iron to form a relatively dmse, non-p ous scale that can significantly reduce continued coirosiraL Other acids, such as oxalic, tend to inraease solution conosivi due to the formation of soluble irtm chelates (Rooney et al., 1996). 

This reaction proceeds rapidly in acids, but only slowly in alkaline or neutral aqueous media. The corrosion rate of iron in deaerated neutral water at room temperature, for example, is less than 5 p,m/year. The rate of hydrogen evolution at a specific pH depends on the presence or absence of low-hydrogen overvoltage impurities in the metal. For pure iron, the metal surface itself provides sites for H2 evolution hence, high-purity iron continues to corrode in acids, but at a measurably lower rate than does commercial iron. 

For more than a century, a number of different aluminum alloys have been commonly used in the aircraft industry These substrates mainly contain several alloying elements, such as copper, chromium, iron, nickel, cobalt, magnesium, manganese, silicon, titanium and zinc. It is known that these metals and alloys can be dissolved as oxides or other compounds in an aqueous medium due to the chemical or electrochemical reactions between their metal surfaces and the environment (solution). The rate of the dissolution from anode to cathode phases at the metal surfaces can be influenced by the electrical conductivity of electrolytic solutions. Thus, anodic and cathodic electron transfer reactions readily exist with bulk electrolytes in water and, hence, produce corrosive products and ions. It is known that pure water has poor electrical conductivity, which in turn lowers the corrosion rate of materials however, natural environmental solutions (e g. sea water, acid rains, emissions or pollutants, chemical products and industrial waste) are highly corrosive and the environment s temperature, humidity, UV light and pressure continuously vary depending on time and the type of process involved. ... 

A similar method of test was used at the International Nickel Company s Corrosion Laboratory at North Carolina. The specimen discs are mounted on insulated vertical spindles and submerged in sea-water, which is supplied continuously to the tank in which the specimens are immersed. The maximum peripheral speed of the spinning disc is about 760cms , and the characteristic pattern of attack is shown in Fig. 19.3a. Studies of variation of depth of attack with velocity indicate that at low velocities (up to about 450 cm s ) alloys such as Admiralty brass, Cu-lONi and cupro-nickel alloys containing iron maintain their protective film with a consequent small and similar depth of attack for the diflferent alloys. At higher velocities the rate increases due to breakdown of the film. 

In plain tinplate cans for acid foods, tin provides cathodic protection to steel (3,4). The slow dissolution of tin prevents steel corrosion. Many investigators (5-1I) have defined this mechanism in detail and have shown that the tin dissolution rate is a function of the cathodic activity of the base steel, the steel area exposed through the tin and the tin-iron alloy layers, and the stannous ion concentration. Kamm et al. showed that control of the growth of the tin—iron alloy layer provides a nearly continuous tin-iron alloy layer and improves the corrosion resistance of heavily coated (over 45 X 10"6 in. tin) ETP for mildly acid food products in which tin provides cathodic protection to steel (12). The controlled tin-iron alloy layer reduces the area of steel exposed to the product. ETP with the controlled alloy is designated type K, and since 1964, 75 type K ETP has been used to provide the same protection as 100 ETP provided previously (13). 

One of the most ingenious ways in which corrosion is inhibited is to strap a power pack to each leg (just above the level of the sea) and apply a continuous reductive current. An electrode couple would form when a small portion of the iron oxidizes. The couple would itself set up a small voltage, itself promoting further dissolution. The reductive current coming from the power pack reduces any ferric ions back to iron metal, which significantly decreases the rate at which the rig leg corrodes. 

At the anodes, iron is dissolved and an oxide deposited. Electrons travel from anode to cathode within the metal, and OH ions travel from cathode to anode through the solution with which it is in contact. For these processes to occur, a continuing source of oxygen is needed, and the surface of the metal must remain wet. If pH is above about 11.5 (P48) and Cl" is absent, the oxide is deposited as a thin protective film which is virtually continuous, and the rate of attack is so low as to be insignificant. The iron is said to be in a passive condition. At a lower pH, an oxide or oxyhydroxide is deposited in an incoherent form, and corrosion is rapid. 

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