Temperature effects corrosion rate

Figure 3.3. Effect of temperature on corrosion rates of steels in crude oil containing sulfur [121.

The effect of temperature on corrosion rate is influenced by the following factors [188] ... 

In addition to the direct effect of film temperature on corrosion rate, an indirect effect has been observed in the heating of some foods and chemicals, in which insulating solid corrosion films form on different metals. By raising the metal surface temperature, these films may, when pervious, lead to further corrosion. 

Based upon the electrodics, the exchange current density is related to the activation energy (16. pp.ll52). For an Arrhenius relation for the effect of temperature upon corrosion rate, AG and AG j are analogous to activation energy and equal 6.44 kcal/mol and 4.30 kcal/mol, respectively. The literature indicates activation energies for mass transfer limited processes range between 1 and 3 kcal/mol and for reaction limited between 10 and 20 kcal/mol (12). Based upon this criteria, corrosion of the 304 S.S. in pure water in the experimental system may lie between the mass transfer and reaction rate limited cases. 

Both the H2S concentration over the range of 0-1.0 v/o of the CGA gas and the temperature controlled the measured corrosion rates. Figure 1 illustrates the effect of temperature on corrosion rates of several alloys and coatings in the CGA gas containing 1 v/o H2S. Alloys AISI 309, AISI 310, and IN-800 demonstrate a clear temperature dependence of total oxidation-corrosion in 1000 hr. The 309 alloy had a scatter band of 5 to 125 mils total metal loss for four specimens at 1650°F. This is typical of borderline alloys that undergo time-dependent transitions to accelerated corrosion rates. Total corrosion of aluminized 310 and 800 was relatively unaffected by temperature over the range of 1500°-1800°F for 1000 hr exposures. 

Most effects of elevated temperatures are adverse to corrosion inhibition. High temperatures increase corrosion rates (about double for a 15°C rise at room temperature), and they decrease the tendency of inhibitors to adsorb on metal surfaces. Precipitate-forming inhibitors are less effective at elevated temperatures because of the greater solubility of the protective deposit. Thermal stability of corrosion inhibitors is an important consideration at high temperatures. Polyphosphates, for example, are hydrolyzed by hot water to form orthophosphates that have little inhibitive value. Most organic compounds are unstable above about 200°C (see Table 17.1) hence, they may provide only temporary inhibition at best. 

In-situ testing is also performed in the field to determine the effectiveness of a water treatment system, the performance of difference alloys, and the effects of var5dng operating parameters such as flow rate and temperature. Uniform corrosion rates, t5fpes of corrosion, and pitting tendencies are identified with in-situ testing. 

The rate of corrosion is controlled by the rate of dissolved oxygen reaction on the surface, which is controlled by the rate of transport of dissolved oxygen to the surface. Similar mass transfer mechanisms control the rate of transfer of fouling ions to the surface. In a system containing a corrosion inhibitor, the transfer of inhibitor to the surface is also controlled by the same factors. Once the reactants are at the surface, reaction rates are affected by temperature. Temperature also affects the properties of the fluid film in contact with the surface. For example, viscosity typically decreases in the film on the surface of a hot wall, facihtat-ing reactant transfer. It is difficult to generalize about the affect of temperature on corrosion rates in a system treated with corrosion inhibitor. Increased temperature is likely to accelerate the corrosion inhibition reactions as well as the corrosion reaction. The net change in corrosion rate could be either an increase or a decrease, depending upon the effectiveness of the corrosion inhibition treatment. 

Expxrsure tempjerature— In general, corrosion rates increases with temperature. However, corrosion rate/temprerature relationships can be strongly influenced by system geometry and impurity effects. 

The overall effect of temperature on corrosion rates is complex. During longterm exposure in a temperate climatic zone, temperature appears to have little or no effect on the corrosion rate. As the temperature increases the rate of corrosive attack increases as a result of an increase in the rate of electrochemical and chemical reactions as well as the diffusion rate. Consequently, under constant humidity conditions, a temperature increase will promote corrosion. Conversely, an increase in temperature can cause a decrease in the corrosion rate by causing a more rapid evaporation of the surface moisture film created by rain or dew. This reduces the time of wetness, which in turn reduces the corrosion rate. In addition, as the temperature increases, the solubility of oxygen and other corrosive gases in the electrolyte film is reduced. 

Stainless steels, on the other hand, develop a relatively thick tarnish film with a thin, powdery surface film. Both films have a nominal composition of where M represents iron, nickel, or chromium. The corrosion rate of stainless steel is not greatly affected by temperature in the range of 260 to 400°C, and it does not exhibit the marked effect of temperature on corrosion rate that is characteristic of Zircaloys. 

For diffusion controlled corrosion reactions e.g. dissolved oxygen reduction, and the effect of temperature which increases diffusion rates, then by substituting viscosity and the diffusion coefficients at appropriate temperatures into the Reynolds No. and Schmidt No., changes in corrosion rate can be calculated. 

By substituting the appropriate values for viscosity and diffusion at various temperatures, they found that corrosion rates could be calculated which were confirmed by experiment. The corrosion rates represent maxima, and in real systems, corrosion products, scale and fouling would reduce these values often by 50%. The equation was useful in predicting the worst effects of changing the flow and temperature. The method assumes that the corrosion rate is the same as the limiting diffusion of oxygen at least initially this seems correct. 

In contrast to the influence of velocity, whose primary effect is to increase the corrosion rates of electrode processes that are controlled by the diffusion of reactants, temperature changes have the greatest effect when the rate determining step is the activation process. In general, if diffusion rates are doubled for a certain increase in temperature, activation processes may be increased by 10-100 times, depending on the magnitude of the activation energy. 

Dissolved oxygen reduction process Corrosion processes governed by this cathode reaction might be expected to be wholly controlled by concentration polarisation because of the low solubility of oxygen, especially in concentrated salt solution. The effect of temperature increase is complex in that the diffusivity of oxygen molecules increases, but solubility decreases. Data are scarce for these effects but the net mass transport of oxygen should increase with temperature until a maximum is reached (estimated at about 80°C) when the concentration falls as the boiling point is approached. Thus the corrosion rate should attain a maximum at 80°C and then decrease with further increase in temperature. 

A striking example of the interaction of solution velocity and concentration is given by Zembura who found that for copper in aerated 0-1 N H2SO4, the controlling process was the oxygen reduction reaction and that up to 50°C, the slow step is the activation process for that reaction. At 75 C the process is now controlled by diffiision, and increasing solution velocity has a large effect on the corrosion rate (Fig. 2.5), but little effect at temperatures below 50 C. This study shows how unwise it is to separate these various... 

The overall effect is that corrosion is usually more rapid at higher temperatures, the corrosion products being often more objectionable in nature. There are, however, exceptions to this generalisation and the increased rate... 

Where a large collection of data exists then it may be effectively condensed in the form of diagrams. A popular method is the use of iso-corrosion rates plotted on co-ordinates of temperature and concentration for one material and one chemical. Because of the large amount of data on the common acids there are many examples of this type of diagram, e.g. the work of Berg who has chosen metals and alloys that are readily available. He has excluded many metals and alloys on the grounds that they are either Non-resistant or can be substituted by cheaper materials. ... 

In the main there exists, for each system of a chemical in contact with those metals and alloys that rely on a passive film, the possibility of an increase in corrosion rate with increasing concentration but reaching a maximum and followed by a decrease in rate. If the concentration when this maximum is reached is low, then the chemical is inhibitive . The effect of temperature on corrosion is dependent on the position of the maximum concentration. For many chemical/metal systems this maximum may be at a temperature... 

The ferritic steels may also undergo intercrystalline corrosion as a result of grain boundary carbide formation. In the normal softened state (treated i 800 C) the carbon is largely precipitated and the ferrite composition homogenised so that further heating at lower temperatures has no adverse effect. During solution treatment above 950 C, however, carbon is redissolved. Sensitisation can then occur at lower temperatures but the rate is so rapid that it can only be suppressed by very rapid cooling which is not practically feasible. Thus weld decay is very possible in service unless a remedial... 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Figure 4.35 illustrates the effect of temperature on the rate of development of pitting, measured as a corrosion current in an acidic solution containing Cl it is seen that quite small increments in temperature have large effects. The influence of temperature is of considerable significance when metals and alloys act as heat transfer surfaces and are hotter than the corrosive environment with which they are in contact. In these circumstances. 

The effect of temperature on the corrosion of zinc in water It is found that the temperature has a marked effect on the rate at which zinc corrodes in water. The corrosion rate in distilled water reaches a maximum in the temperature range 65-75°C. This variation in the corrosion rate with temperature is attributed to changes in the nature of the protective film. At lower temperatures the film is found to be very adherent and gelatinous, while at temperatures around 70°C it becomes distinctly granular in character and much less adherent. Above 75°C it again tends to become more adherent and assumes a very compact and dense form. It is believed that the granular coating formed at temperatures around 70°C is more porous... 

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