Protective scales, corrosion

The corrosion rate of steel in carbonic acid is faster than in hydrochloric acid Correlations are available to predict the rate of steel corrosion for different partial pressures of CO2 and different temperatures. At high temperatures the iron carbonate forms a film of protective scale on the steel s surface, but this is easily washed away at lower temperatures (again a corrosion nomogram is available to predict the impact of the scale on the corrosion rate at various CO2 partial pressures and temperatures). 

The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

Normally, a layer of FeS scale, produced in the first reaction, protects the interior of the pipe or vessel. However, in the second reaction, cyanide removes the FeS protective scale exposing more free iron to react with H2S and releasing more hydrogen. Without protection, this cycle continues until blistering, cracking, and eventual total corrosion of equipment occurs. 

With insufficient carbon dioxide of type 3 (and none of type 4) the water will be supersaturated with calcium carbonate and a slight increase in pH (at the local cathodes) will tend to cause its precipitation. If the deposit is continuous and adherent the metal surface may become isolated from the water and hence protected from corrosion. If type 4 carbon dioxide is present there can be no deposition of calcium carbonate and old deposits will be dissolved there cannot therefore be any protection by calcium carbonate scale. 

Impingement attack Copper may occasionally suffer this form of attack in systems where the speed of water flow is unusually high and the water is one that does not form a protective scale, e.g. a soft water containing appreciable quantities of free carbon dioxide . Ball valve seatings may also suffer an erosive type of attack. The corrosion of ball valves, including the effect of chlorination of the water, has been studied by several workers... 

It has been noted that the total current required to protect large structures can be substantial even in mildly corrosive environments. In seawater, for example, an initial current in the region of 200mA/m for bare steel might well be required in the North Sea. This is because the relatively high oxygen concentration and the tide and wave action all contribute to a facile cathodic reaction. Fortunately this current diminishes with time. The reason for this is the protective scale on the steel surface which forms during cathodic protection by decomposition of the seawater. 

Immersion in aqueous media open to air Solutions in which tin is cathodic to steel cause corrosion at pores, with the possibility of serious pitting in electrolytes of high conductivity. Porous coatings may give satisfactory service when the corrosive medium deposits protective scale, as in hard waters, or when use is intermittent and is followed by cleaning, as for kitchen equipment, but otherwise coatings electrodeposited or sprayed to a sufficient thickness to be pore-free are usually required. 

Erosion-corrosion. Generally, all types of corrosive media can cause erosion-corrosion, including aqueous solutions, organic media, gases, and liquid metals. The corrodent can be a bulk fluid, a film, droplets or a substance adsorbed on or absorbed on another substance. For example, hot gases may oxidize a metal at high velocity and blow off an otherwise protective scale. Solid suspensions in liquids (slurries) are particularly destructive from the standpoint of erosion corrosion.16 31... 

Both neutralizers are injected in the fractionator overhead line in order to be present when the dew point of hydrochloric acid in solution is reached. It is important to use a quill to inject neutralizers or inhibitors because drip injection can cause dissolution of the protective scale on the inside of the pipe, which can result in corrosion and erosion in that area. Often, however, neutralization is not accomplished, and severe corrosion from hydrochloric acid still occurs at the dew point. The pH is controlled at the overhead receiver water draw because dew point pH measurement is not feasible. One method of controlling the dew point pH is to recycle water from the drum to the overhead line. This water buffers the condensate at the hydrochloric acid dew point and also provides water in which the ammonia can dissolve. 

In Chapter 8 crystallisation and scale formation are discussed and the effect of pH was demonstrated as being a factor in deposit formation. Furthermore the equilibrium of ions in solution is likely to be affected by the presence of electric currents and the current density. As a consequence, a characteristic feature of cathodically protected metallic surfaces in contact with solutions containing mineral ions, notably sea water, is the formation of deposits or cathodic protection scale. The use of controlled scale formation is mentioned as a method of corrosion control in Chapter 14. Cox [1940] proposed the deliberate formation of calcium deposits on steel by the imposition of large cathodic currents to act as anticorrosive self healing layer. 

In petroleum refineries, process streams containing hydrogen also frequently contain hydrogen sulfide. This causes sulfidic corrosion. You know from experience that increasing the chromium content of a steel increases its resistance to corrosion by high-sulfur crudes. However, do not jump to the conclusion that chromium alloying always improves resistance to sulfidic corrosion. It does so if the operation is dirty, as it usually is in crude streams, or if the corrodents are elemental sulfur or sulfur compounds that do not decompose to release hydrogen sulfide. This increased resistance to sulfur corrosion depends on formation of a protective scale. With such scales, the corrosion rate is parabolic — it decreases with exposure time. 

Deposition effects of rare earth oxides on the surfaee of iron and stainless steels have also been reported [58]. The eorrosion rate eonstants decreased significantly by the coating in corrosion tests under isothermal eonditions. In thermal cyclic conditions, protective scale spallation completely disappeared for eoated samples. The rare earth effect is more remarkable with elements located on the left part of the lanthanide series (lighter rare earths). For example, eeria coatings strongly modify the microstructure and texture of the wustite (FeO) scale formed during low pressure oxidation of pure iron [59]. Cerium is located in the wustite matrix as a CeFeOs phase which dissolves in FeO in time. 

Sulphidation reactions follow a similar series of kinetic phenomena as has been observed for oxidation. Unfortunately, few studies have been made of the basic kinetic phenomena involved in sulphidation reactions at high temperature. Similarly, the volatile species in sulphate and carbonate systems are important in terms of evaporation/condensation phenomena involving these compounds on alloy or ceramic surfaces. Perhaps the best example of this behaviour is the rapid degradation of protective scales on many alloys, termed hot corrosion , which occurs when Na2S04 or other salt condenses on the alloy. 

This degradation is self sustaining. Metal ions go into solution at the alloy-salt interface and precipitate as a non-protective solid in the salt, but the metal that dissolves and reprecipitates is the more noble metal rather than the elements that would form the protective scale in the absence of hot corrosion. 

This type of behaviour has been observed for the nickel-chromium alloy system. In this work, pure nickel, Ni-20 wt% Cr and Ni-30 wt% Cr alloys were exposed to erosion-corrosion at 700 and 800 °C. An erosive stream, loaded at 400 mg min of 20 iJiva alumina particles and flowing at 75 and 125 m s , was used to impact normally on the specimens. Under simple oxidation in air at these temperatures, the alloys both developed protective scales of chromia and showed very low rates of... 

In the strict sense, the passive mode implies that the newly formed phase is retarding the process, that is corrosion is slowed down with time protective scale), but there are cases in which the scale is nonprotective a scale with cracks, low viscosity, or foamy texture may not hinder the access of the corrosive agent to the substrate. 

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