Rust layers

In oxygenated water of near neutral pH and at or slightly above room temperature, hydrous ferric oxide [FelOHla] forms on steel and cast irons. Corrosion products are orange, red, or brown and are the major constituent of rust. This layer shields the underl3dng metal surface from oxygenated water, so oxygen concentration decreases beneath the rust layer. 

More reduced forms of oxide are present beneath the rust layer. Hydrous ferrous oxide (Fe0 nH20), that is, ferrous hydroxide [Fe(OH)2l... 

Figur 3.2 Incipient rust layer on steel in oxygenated water. (Courtesy of National Association of Corrosion Engineers, Corrosion 91 Paper No. 84 by H. M. Herro.)...

As rust accumulates, oxygen migration is reduced through the corrosion product layer. Regions below the rust layer become oxygen depleted. An oxygen concentration cell then develops. Corrosion naturally becomes concentrated into small regions beneath the rust, and tubercles are born. 

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

Figure 5.6 Rust layer on carbon steel mill water supply pipe. Note the partially exfoliated region at the bottom of the photo.

The distinguishing feature of the behaviour of the slow-rusting low-alloy steels is the formation of this protective rust layer. Corrosion in conditions where it cannot form is little different from that of unalloyed steel, although the particular alloying elements present will have some influence on the actual rate at which corrosion occurs. 

The adsorption isotherms for metallic surfaces are reported in the literature however, an important part of the atmospheric corrosion process takes place under rust layers, which play a decisive role in the long-term course of corrosion because of its sorption capacity for water. The influence of the chloride and sulfate anions has a real effect only when the corrosion products layer is already formed. Thus, the adsorption isotherms of the steel corrosion products formed in different atmospheres were determined. 

Mossbauer spectroscopic study of reactions within rust layers. Corrosion Sd. 29 1329—... 

You find that the rust layer from a spot on your car fender is 0.3 mm thick 6 months after the paint was ehipped off If the sheet metal is 1 mm thick, when do you have to sell it to be able to tell the buyer it has no rust holes ... 

A rusted anode appears as a raised orange to brown upper layer occupying more volume than the original iron surface. This expansion loosens the rusted layer to expose even more surface to corrosive attack. 

On top, the red rust layer explains the absence of hydrodynamic effects after two days of immersion. This layer, which is an electronic insulator but an ionic conductor, does not play any role on the kinetics. 

An effect not considered in the above models is the added resistance, caused by fouling, to solute back-diffusion from the boundary layer. Fouling thus increases concentration polarization effects and raises the osmotic pressure of the feed adjacent to the membrane surface, so reducing the driving force for permeation. This factor was explored experimentally by Sheppard and Thomas (31) by covering reverse osmosis membranes with uniform, permeable plastic films. These authors also developed a predictive model to correlate their results. Carter et al. (32) have studied the concentration polarization caused by the build-up of rust fouling layers on reverse osmosis membranes but assumed (and confirmed by experiment) that the rust layer had negligible hydraulic resistance. 

FIGURE 22.32 Anion-selective and cation-selective rust layers on corroding metals in chloride solution ... 

As the local concentration of chloride ions increases, the occluded solution under the anion-selective layer will be acidified and the passive film on metals will break down giving rise eventually to localizing metal corrosion under the layer. It is thus obvious that the presence of an anion-selective rust layer accelerates the corrosion of the underlying metal. In order to prevent such accelerated corrosion under an anion-selective layer, we need to reduce the anion-selective nature of the layer by some way such as the adsorption of multivalent anions on the layer. 

With a cation-selective rust layer, the anodic ion transport across the mst layer carries mainly hydrated protons migrating outward from an occluded solution into the solution bulk as shown in Figure 22.32b. No accumulation of chloride ions and hydrogen ions is thus expected to occur in the occluded solution. Instead, the outward migration of hydrogen ions leads to the basification of the occluded solution, where the precipitation of corrosion products will then be accelerated. 

Furthermore, the electroosmotic outward flow of water molecules, which follows the anodic hydrogen ion transport, counteracts the inward diffusion of water molecules into the occluded solution. The dehydration of the occluded solution will then occur as the corrosion progresses. Since metal dissolution requires water molecules for metal ions to hydrate, the depletion of water molecules will finally result in the deceleration of metal corrosion. The cation-selective rust layer, therefore, will be preventive of the corrosion of underlying metals. 

Let us consider a composite rust layer consisting of an anion-selective inner layer and a cation-selective outer layer as shown in Figure 22.33a. A bipolar ion-selective layer of this type may be realized when the outer part of an anion-selective layer is made cation-selective by the adsorption of multivalent anions. As with the electronic rectifier of p-n junction semiconductors, a bipolar junction consisting of an anion-selective layer and a cation-selective layer rectifies the ionic current across the bipolar layer. The ion transfer current in the anodic direction is thus suppressed across the bipolar layer as shown in Figure 22.34, where the anodic ion transport current is seen to be restricted across a bipolar ferric hydroxide membrane in sodium chloride solution [70]. 

FIGURE 22.33 Bipolar ion-selective rust layers on metals in aqueous solution (a) anodic ion transport suppressed by the backward bipolar ion selectivity and (b) profile of electrostatic potential across a backward bipolar ion-selective rust layer [65]. 

In the presence of chloride ions, a local breakdown of rust layers makes an anode channel for localized corrosion of underlying steel, and the chloride ions tend to accumulate in the channel as the anodic metal dissolution progresses. Assuming the ferrous chloride concentration at 1 mol dm 3 in the anode channel, we obtain the proton level at pH 4.75, where no ferrous hydroxide precipitation is expected to occur because of its solubility greater than 1 mol dm 3. The hydrated ferrous chloride produced by corrosion is then oxidized by air-oxygen in the anode channel ... 

This oxidation generates hydrochloric acid and makes the anode channel acidified. As the acidification progresses, the rate of the air-oxidation of hydrated ferrous ions steeply decreases and the solubility of ferric hydroxide increases at the same time. The anode channel, as a result, holds a mass of hydrated ferrous and ferric ions, which gradually leaks out of the rust layer. 

A relatively simple method is to dissolve out the chloride ions by immersion in a suitable solvent. Water has been used with the water being changed every month until no further chlorides are detected. This can take up to 5 years for marine artefacts with high levels of chloride buried within deep rust layers. Moreover, the metal will continue to corrode, while the artefact is immersed in the water for this length of time. By altering the pH of the solution it may be possible to dissolve out the chlorides without corroding the metal. This is achieved by forming a thin, passive film approximately 10 nm ( 10 9m) thick... 

The principle of this method of conservation is to immerse the artefact in a tank containing a suitable solution. The chloride ion dissolves from the rust him into the solution that is changed, initially, every week and subsequently every month. The chloride content of the solution is analyzed at the end of each changeover. The process is continued until there is no more chloride detected. At this point, the artefact is deemed to be conserved. This can take up to 5 years for marine artefacts with high levels of chlorides buried within deep rust layers. Even after this length of time, one is not absolutely certain that all the deleterious ions have been removed from the rust/metal interface. 

Some conservators have alternated between tanks of boiled and cold water for their artefacts. They claimed that the expansion and contraction of the artefact, will assist in the removal of the deeply-buried chlorides from the rust layers. One must be careful that this does not cause the rust to spall off the underlying metal due to the difference in expansion coefficients between the two classes of materials. 

The treatment of small ferrous artefacts in a 0.05 M lithium hydroxide dissolved in methanol or ethanol has its advocates, particularly in France. The chlorides present in the rust layers react with lithium hydroxide to form lithium chloride that dissolves in the alcohol phase. Any of the hydroxide left on the metal surface combines with any carbon dioxide to form a solution with pH above 9.5, which maintains any exposed metal in the passive region. Hence, this solution is claimed to cause no corrosion of the underlying metal. The real disadvantage of this solution is that any lithium chloride left on the surface of the artefact is very hygroscopic. Water will form on the surface at a relative humidity above 15% RH and corrosion of the metal will take place. Humidity levels below 15% RH are very difficult to maintain in display cabinets or in storage and is one of the main reasons why this solution has not been more widely employed. 

This reduction involves a 30% decrease in volume, which makes the rust layers more porous and allows the solution to reach the deeply-buried chlorides. The conservation must be carried out in sealed containers as ingress of oxygen into the solution would convert the sulfite to sulfate, and hence no reduction would take place. Some conservators have heated the solutions up to 60°C to speed up the reduction rates, and hence the removal of chloride ions. The two major problems with this particular formulation are that the chloride analysis is rather difficult and is not very efficient for artefacts covered in thick layers of corrosion products. 

Once in dry dock, the outside of the ship was pressure-washed with mains water to remove marine growths attached to the steel plates as well as the loosely adherent corrosion products. This was repeated several times to assist in the removal of chloride ions from the rust layers. Approximately 11 tons of debris were removed from the external structure of the ship by this process. Several sections of the steel plates were found to have very thin areas less than 1 mm... 

Corrosion due to the condensate level maintained in the reboiler often occurs with the condensate outlet scheme. In one case (239), a rust layer on the steam side of the reboiler showed the level at which steam condensate usually ran. 

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