Reducing aqueous corrosion

Additionally, crevice corrosion can be reduced by two techniques used successfully on most aqueous corrosion—chemical inhibition and cathodic protection. However, both these techniques may be cost prohibitive. 

Removing suspended solids, decreasing cycles of concentration, and clarification all may be beneficial in reducing deposits. Biodispersants and biocides should be used in biofouled systems. Simple pH adjustment may lessen precipitation of certain chemical species. The judicious use of chemical corrosion inhibitors has reduced virtually all forms of aqueous corrosion, including underdeposit corrosion. Of course, the cleaner the metal surface, the more effective most chemical inhibition will be. Process leaks must be identified and eliminated. 

Environments are either gases or liquids, and inhibition of the former is discussed in Section 17.1. In some situations it would appear that corrosion is due to the presence of a solid phase, e.g. when a metal is in contact with concrete, coal slurries, etc. but in fact the corrosive agent is the liquid phase that is always present. Inhibition of liquid systems is largely concerned with water and aqueous solutions, but this is not always so since inhibitors may be added to other liquids to prevent or reduce their corrosive effects — although even in these situations corrosion is often due to the presence of small quantities of an aggressive aqueous phase, e.g. in lubricating oils and hydraulic fluids (see Section 2.11). 

Rust formed by atmospheric corrosion is often voluminous (Fig. 18.4) and visually appears as loose orange-brown or black masses. This type of rust is always a mixture of phases and frequently consists of two layers - magnetite at the iron/rust interface (as a result of reduced oxygen supply) with an outer layer of loose lepidocrocite and/ or goethite. Hematite is formed during high temperature aqueous corrosion and is also found in the passive layer (which forms at room temperature). 

At the other extreme, the oxide layers on aluminum, beryllium, titanium, vanadium, chromium, nickel, and tantalum are very insoluble in water at intermediate pH values and do not have easily accessible reduced states with higher solubility. The oxide films on those metals are therefore highly protective against aqueous corrosion. 

Attention to the pH of an aqueous solution may reduce the corrosion potential, but in general it is not feasible for the heat exchanger designer to call for changes in pH, since he or she has to work within the limits of the specification of the heat exchanger. 

Hydrophobic treatment. In principle, reduction in the moisture content of concrete can also reduce the corrosion rate in chloride-contaminated concrete, and thus hydrophobic treatment may be used to control the corrosion rate. Nevertheless, there is insufficient knowledge about the critical moisture content in the case of chloride-induced corrosion. The greater the content of chlorides and their hygroscopicity, the lower the moisture content must be. The hterature is conflicting in some cases hydrophobic treatments seem to slow down chloride-induced corrosion, in other cases they clearly do not [8]. However, since no precise data are available in this instance, an evaluation must be made of the advisabihty of applying hydrophobic treatments and impermeable coatings that prevent concrete from coming into direct contact with water (but allow aqueous vapour to pass so that the concrete will dry out), on a case-by-case basis. 

For example, it is not uncommon for upstream steel corrosion to produce ferric corrosion products (i.e., Fe ions), which contaminate process streams. Several ppm concentrations or more of any of these species may partially or fully passivate titanium alloys in reducing aqueous media depending on acid concentration and temperature, and should be taken into account when evaluating alloy corrosion resistance. 

An oxide layer is readily formed on many metals when they are made anodic in aqueous solutions. In the case of aluminum, this process is called anodization. It is also referred to as a passive film which reduces the corrosion rate. Such passive films can be thin, from 0.01 pm, and fragile and easily broken. Thus, when steel is immersed in nitric acid or chromic acid and then washed, the steel does not immediately tarnish nor will it displace copper from aqueous CUSO4. The steel has become passive due to the formation of an adhering oxide film which can be readily destroyed by HCl which forms the strong acid FeCU-. 

Note that all of the above reactions are similar in one respect—they consume electrons. All corrosion reactions are simply combinations of one or more of the above cathodic reactions, together with an anodic reaction similar to Eq. (3.10). Thus, almost every case of aqueous corrosion can be reduced to these equations, either singly or in combination. 

The in-plle and out-of-plle aqueous corrosion and hydrogen pick-up of zirconium alloys used as reactor pressure tube materials Is discussed particularly in relation to differing behaviour under oxidizing (neutral) and comparatively reducing environments (ammonia). The materials selection and chemical aspects of the moderator circuit are outlined. 

Abstract To improve the corrosion resistance of magnesium (Mg) alloys, surface modification is applied in an attempt to produce a corrosion resistant barrier. In this chapter, a new method to deposit aluminum (Al) film on an Mg alloy surface is demonstrated. Electrodeposition of Al on AZ91D Mg alloy, using an acidic aluminum chloride-l-ethyl-3-methylimidazolium chloride ionic liquid (AICI3-EMIC), has been shown to be feasible. The existence of Al coating can cause a substantial increase in corrosion resistance, reducing the susceptibility of Mg alloy to aqueous corrosion. 

The results of field tests on 250 units over an 18-month period have been summarized by Fbwler (1964). Calcium chloride has also been proposed for hydrate prevention in natural-gas gathering lines. In this application an aqueous solution is injected near the well head and collected at the downstream point after gas cooling has occurred. In tests reported in Russian literature (Andiyudichenko and Vasilchenko, 1963), the process was found to be very effective. However, purging of the solutions with natural gas prior to injection was found necessary to reduce their corrosive action. 

To minimize aqueous chloride corrosion in the overhead system of crude towers, it is best to keep the salt content of the crude oil charge as low as possible, about 4 ppm. Another way to reduce overhead corrosion would be to inject sodium hydroxide into the crude oil, downstream of the desalter. Up to 10 ppm caustic soda can usually be tolerated. 

As an example of SAMs, eorrosion inhibition by self-assembled films formed by adipic acid (AA) molecules on caibon steel surfaces is discussed below. SAMs of AA were formed on iron oxide/caibon steel surfaces by the immersion coating method. The metal was immersed in an aqueous solution containing 60 ppm of Cl (to initiate the corrosion process and the formation of iron oxide) in the absence and presenee of adipic acid. The formation, uniformity, ordering and bonding of the monolayers accompUshed by the immersion method have been evaluated by FTIR and AFM. The electrochemical properties of the unmodified and modified caibon steel surfaces were characterized by polarization study and EIS analysis to test the abiUty of the monolayer to reduce the corrosion of the surface. 

The general conclusion reached was that aqueous homogeneous reactors are potentially very low-cost plutonium producers however, considerable development work remains before large-scale reactors can be constructed. The major problem is due to the corrosiveness of the relatively concentrated uranyl sulfate solutions used in such reactors, which requires that all the equipment in contact with high-temperature fuel be made of titanium, or carbon-steel lined, or clad with titanium. The development of suitably strong titanium alloys, bonding methods, or satisfactory steel-titanium joints has not yet proceeded sufficiently to consider the construction of full-scale plutonium producers. Alternate approaches, such as the addition of Li2S04 to reduce the corrosiveness of stainless steel by the fuel solution (see Chap. 5), show promise but also require further development. 

Metals. Most metals react with aqueous HCl foUowing equation 22. The reaction rate is dependent on the concentration of the acid, oxidi2ing, reducing, or complexing agents, and corrosion inhibitors, in addition to the metallurgical characteristics of the material and the prevailing hydrodynamic conditions (see Corrosion and corrosion control). 

Another attractive commercial route to MEK is via direct oxidation of / -butenes (34—39) in a reaction analogous to the Wacker-Hoechst process for acetaldehyde production via ethylene oxidation. In the Wacker-Hoechst process the oxidation of olefins is conducted in an aqueous solution containing palladium and copper chlorides. However, unlike acetaldehyde production, / -butene oxidation has not proved commercially successflil because chlorinated butanones and butyraldehyde by-products form which both reduce yields and compHcate product purification, and also because titanium-lined equipment is required to withstand chloride corrosion. 

The titanium oxide film consists of mtile or anatase (31) and is typically 250-A thick. It is insoluble, repairable, and nonporous in many chemical media and provides excellent corrosion resistance. The oxide is fully stable in aqueous environments over a range of pH, from highly oxidizing to mildly reducing. However, when this oxide film is broken, the corrosion rate is very rapid. Usually the presence of a small amount of water is sufficient to repair the damaged oxide film. In a seawater solution, this film is maintained in the passive region from ca 0.2 to 10 V versus the saturated calomel electrode (32,33). 

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