Corrosion increases with increasing

Materials of Construction. Resistance of alloys to concentrated sulfuric acid corrosion increases with increasing chromium, molybdenum, copper, and silicon content. The corrosiveness of sulfuric acid solutions is highly dependent on concentration, temperature, acid velocity, and acid impurities. An excellent summary is available (114). Good general discussions of materials of construction used in modem sulfuric acid plants may be found in References 115 and 116. More detailed discussions are also available (117—121). For nickel-containing alloys Reference 122 is appropriate. An excellent compilation of the relatively scarce literature data on corrosion of alloys in liquid sulfur trioxide and oleum may be found in Reference 122. 

The corrosion resistance of selected Si3N4 ceramics in NaOH is given in Fig. 40. The attack by bases is less pronounced than by acids. The extent of corrosion increases with increasing temperature and concentration of the bases [524], Also, in bases the intergranular films are not attacked as strongly as the triple points. In NaOH solutions often a linear dependence of the weight loss on time is reported. Materials which are less stable in acids are more stable in bases. This can be explained by the stability of the grain boundary phase [510],... 

Iron. Ruthenium. Osmium. Iron (and steel) are rapidly corroded by PH3 in the presence of air and humidity. The rate of corrosion increases with increasing temperature and relative humidity, and probably involves two or more stages. The initial formation of one or more acids of phosphorus is followed by an attack of these acids on iron [82]. 

This is an irreveisible reaction and, consequently, its rate is affected only by temperature and the concentrations of the species on the left side of the equation. Therefore, the rate of reaction 3-3 (and of corrosion) increases with increasing hydrogen ion concentration (i.e., decreasing pH), with increasing temperature, and, because the reaction is electrochemical, with increasing conductivity of the liquid medium. 

The increase in hydrogen ion concentration fiom reaction 3-4 accelerates corrosion by reaction 3-3. As would be expected from reactions 3-3 and 3-4, the rate of corrosion increases with increased CO2 concentration in the water (or increased CO2 partial pressure in the gas phase). 

Aluminium of the series 1000 (including refined aluminium 1199), is also prone to intercrystalline corrosion [ 15]. In water at a temperature above 60-70 °C, the sensitivity to intercrystalline corrosion increases with increasing purity of the metal. Intercrystalline corrosion is caused by the presence of AIFes intermetallics at the grain boundaries [16]. 

Other authors have attributed the improved corrosion resistance with increasing Cr content with the increasing tendency of the oxide to become more disordered [69]. This would then suggest that an amoriDhous oxide film is more protective than a crystalline one, due to a bond and stmctural flexibility in amoriDhous films. 

Both types of babbitt ate easily cast and can be bonded rigidly to cast iron, steel, and bton2e backiags. They perform satisfactorily when lubricated against a soft steel shaft, and occasional corrosion problems with lead babbitt can be corrected by increasing the tin content or shifting to high tin babbitt. 

Where waters have a stabiHty index of 6.0 or less, scaling increases and the tendency to corrode decreases. Where the stabiHty index exceeds 7.0, scaling may not occur at all. As the stabihty index rises above 7.5 or 8.0, the probabiHty of corrosion increases. Use of the LSI together with the Stabihty Index contributes to more accurate prediction of the scaling or corrosive tendencies of a water. 

The mechanism of subcritical crack growth is the reaction of the corrosive medium with highly stressed bonds at the crack tip. In siUca, in the absence of stressed bonds, the rate of the reaction between the bonds and corrosive media such as water is very low. The introduction of strain energy into crack tip bonds increases the activity of the bond. For siUca glass in water, attack and bond breakage occurs by the following reaction (47) ... 

As to the effec t of time, there is no universal law that governs the reaction for all metals. Some corrosion rates remain constant with time over wide ranges, others slow down with time, and some alloys have increased corrosion rates with respect to time. Situations in which the corrosion rate follows a combination of these paths can develop. Therefore, extrapolation of corrosion data and corrosion rates should be done with utmost caution. 

In general, as the corrosiveness of the environment increases, the rate of crack growth also increases. Environmental factors constituting corrosiveness vary with the metal under consideration. 

Alter the chemistry of the common fluid to render it less conductive and/or less corrosive. Generally, water corrosivity increases with an increase in temperature and oxygen content and a decrease in pH. Inhibitors may he effective. Note that in mixed-metal systems, higher dosages of inhibitors may be required than would be necessary in single-metal systems in the same environment. 

Oxygen solubility decreases almost linearly with increasing temperature but the diffusion rate increases exponentially. This leads to a slight increase in corrosion rate with increasing temperature although in Eq. (4-6) the factor is assumed to be greater. For this reason an increase in corrosion rate of about 1.5 times is considered in tropical waters compared with the North Atlantic. 

Critical relative humidity The primary value of the critical relative humidity denotes that humidity below which no corrosion of the metal in question takes place. However, it is important to know whether this refers to a clean metal surface or one covered with corrosion products. In the latter case a secondary critical humidity is usually found at which the rate of corrosion increases markedly. This is attributed to the hygroscopic nature of the corrosion product (see later). In the case of iron and steel it appears that there may even be a tertiary critical humidity . Thus at about 60% r.h. rusting commences at a very slow rate (primary value) at 75-80% r.h. there is a sharp increase in corrosion rate probably attributable to capillary condensation of moisture within the rust . At 90% r.h. there is a further increase in rusting rate corresponding to the vapour pressure of saturated ferrous sulphate solution , ferrous sulphate being identifiable in rust as crystalline agglomerates. The primary critical r.h. for uncorroded metal surfaces seems to be virtually the same for all metals, but the secondary values vary quite widely. 

This is presumably an estimated average curve, as no numerical data are quoted, and it may be assumed to refer to bare steel. This conclusion is not supported by the results of Volkening, whose main interest was in the effect of chlorination and who shows that although corrosion increased with velocity of chlorinated sea water, when plain sea water was used velocity had little effect. There can be no doubt that painting will very much reduce the effect of water speed, as also will marine fouling or slime. 

Type 1. Increasing corrosion rate with increasing concentration and temperature In this case the equation obeyed is... 

Example I. Hard lead (antimoniacal) can be used in sulphuric acid to quite high concentration but it displays an increasing corrosion rate with increasing temperature and concentration. Relationships are complex, but the general form of the equation may be used ... 

Fig. 2.26 Type 4 behaviour decreasing corrosion rate with increase in temperature, e.g. Fe-17-Cr in H,POj. Iso-corrosion lines at 0.1 g m" h ...

Increasing corrosion rate with increasing concentration and temperature... 

Decrease in corrosion rate with increase in temperature... 

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... 

Austenitic cast irons show particularly good corrosion resistance in alkaline environments, even better than that shown by low alloy cast irons. The resistance to corrosion improves with increasing nickel content (Fig. 3.51),... 

Some of the investigations involving electrochemical measurements have been concerned with relating easily determined quanities such as corrosion potential and corrosion current with the behaviour of a material in corrosion fatigue, so that this behaviour can be rapidly assessed without the necessity of the laborious collection of data which was the feature of McAdam s approach. Endo and Komai have derived an expression relating the increase... 

Additionally, there may be COj, HjS or bacteria present, all of which substantially increase the corrosivity of formation waters. Furthermore, whilst in a young oil and gas well the levels of produced formation waters (termed watercut ) may well be very low, at later stages of maturity, the watercut may reach values in excess of 90%. Consequently, oil and gas production systems may often be subject to increasing corrosion risk with time. 

Silica is susceptible to attack by all three reagents, the rate of corrosion increasing with temperature and concentration. Hence 5% caustic soda solution can be contained in fused silica at room temperature, but attack becomes significant at pH values greater than 9, as shown in Fig. 18.3. 

Examples of their results [154] are shown in the set of curves in Fig. 13. At a given humidity, the Co concentration increases with T the thermal activation energy is about 0.4 eV. At a given temperature, the corrosion increases with an increase in humidity. As the humidity changes from 30 to 90%, the corrosion rate increases about an order of magnitude. The data allow a calculation of the acceleration factors for a variety of conditions. For example, the acceleration factor for 90°C/90% RH with respect to 30°C/40% RH is calculated to be 150. If the product passes a 2-week exposure to 90 °C/90% RH, the test indicates that it will survive in excess of 6 years at 30 °C/40% RH. The values of the acceleration factors, however, may vary from film to film. 

Ball et al. investigated the effect of carbon surface area on carbon corrosion at 1.2 V for 24 h and found that, for commercial carbon blacks, cumulative carbon corrosion correlated with carbon BET (Brunauer Emmett Teller) area, although when analyzed as specific carbon corrosion (weight of carbon corroded per unit of carbon area), some variation was observed. The effect of Ft on carbon corrosion has also been studied and conflicting results have been reported. Roen, Paik, and Jarvi found that Ft did increase carbon corrosion... 

Using ISOCORRAG, MICAT and Russian data, Tidblad et al [19] showed that the inclusion of temperature among the environmental parameters improves considerably the usefulness of the dose-response functions and should be adapted in the revision of ISO 9223 standard. It is reported an increase in corrosion rate with average air temperature in the range of -15 to 30°C. 

In the presence of a given value of chloride deposition rate, an acceleration of corrosion takes place this acceleration means that corrosion increases with time. At the same time, the acceleration of corrosion caused by chlorides depends on the washing or cleaning effect of rain. Under this condition the following model is proposed ... 

Cadmium is close to lead and mercury as a metal of current toxicological concern.12346 Extraction of lead and zinc ores, which contain cadmium, pollutes the environment with cadmium. The use of cadmium before 1900 was infrequent however, its valuable metallurgical properties, such as resistance to corrosion, increased its use markedly, for example in the manufacture of alloys and as a coating on steel. It is also now widely used in nickel cadmium ( nicad ) batteries. Coal and other fossil fuels contain cadmium which is released into the environment on combustion. 

---