Oxide films erosion-corrosion

Affected metal surfaces will often contain grooves or wave-like marks that indicate a pattern of directional attack. Soft metals, such as copper and aluminum alloys, are often especially prone to erosion-corrosion, as are metals such as stainless steels, which depend on thin oxide films for corrosion protection. 

Erosion/corrosion of the weld bead at the fracture location damaged the aluminum oxide film on the piping, thus allowing the mercury to wet and initiate cracking of the aluminium. 

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... 

Elevated speeds have a marked effect on wear, and this is more pronounced if the solution contains some solid particles in suspension. Aluminum forms films of aluminum nitrate or oxide in fuming nitric acid. At low flow rates there is no attack whereas for speeds greater than 1.22 m s the protective layer is removed and erosion-corrosion occurs more readily.16... 

Piping or plumbing systems made of copper alloys are susceptible to erosion-corrosion in unfavorable fluid flow conditions. Erosion-corrosion can occur when erosive action of the flowing stream removes the protective copper oxide film from the metal surface, and thus exposing the bare metal surface to a corrosive environment (44). 

In several cases, materials for combined erosive and corrosive conditions have been evaluated on the basis of separate erosion and corrosion studies and data, with the consequence that the synergistic effects are left out of the evaluation. Since one or the other of these effects may be large, the conclusions may be quite wrong. For materials fliat usually are passive due to a dense oxide film, such as stainless steels, Wc is by definition very low. But since sand erosion more or less destroys the passive film, the corrosion rate increases strongly and may reach very high values, i.e. the contribution of Wce may be particularly high for these materials. The other synergy effect, Wec, is most pronounced for ceramic-metallic materials in which the metallic phase has inferior corrosion resistance, e.g. for a cemented carbide with a metallic phase of cobalt (WC-Co). 

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

The control of corrosion and erosion of slurry pipelines is covered by Chapter VIII of the ASME Code B31.11. The code correctly points out that in certain cases erosion is an accelerating factor in the internal corrosion of pipelines, by effectively removing scales, oxides, films, and lining. 

The protective oxide film of most metals is subject to being swept away above a critical water velocity. After this takes place, accelerated corrosion attack occurs. This is known as erosion-corrosion. For some metals, this can occur at velocities as low as 2-3 ft/s. The critical velocity for titanium in seawater is in excess of 90 ft/s. Numerous corrosion-erosion tests have been conducted and all have shown that titanium has outstanding resistance to this form of corrosion. 

STEAM. H2O. In laboratory tests under static conditions, alloy 3003 was found to be resistant to pure steam over distilled water at temperatures up to 268 C (514°F). In fact, aluminum alloys exposed to steam at these temperatures had improved resistance to corrosion by other environments because of the increased thickness of the oxide film on the surface. In the same tests, steam at 268°C (514°F) was corrosive. High pressure steam can erode aluminum alloys by impingement corrosion erosion, particularly when the jet of steam is perpendicular to the surface. Aluminum alloy equipment including heat exchangers. dryers, steam jacketed kettles, piping have been used to handle steam in the petroleum, chemical and food processing industries. See also Ref (1) p. 144, (2) p. 778, (4) p. 49, (7) p. 175. 

Nickel increases corrosion resistance by the formation of protective oxide films on the surfaces of the castings. Up to 4% Ni is added in combination with chromium to improve both strength and corrosion resistance in cast iron alloys. The enhanced hardness and corrosion resistance obtained is particularly important for improving the erosion-corrosion resistance of the material. Nickel additions enhance the corrosion resistance of cast irons to reducing acids and alkalies. Nickel additions of 12% or greater are necessary to optimize the corrosion resistance of cast irons. 

Table 1 of a paper by Murr (2) lists problems and/or concerns related to specific interface materials and specific components of SECS. In Table 2 of the same work, he related topical study areas and/or research problems to S/S, S/L, S/G, L/L, and L/G interfaces. It is also useful to divide interface science into specific topical areas of study and consider how these will apply to interfaces in solar materials. These study areas are thin films grain, phase, and interfacial boundaries oxidation and corrosion adhesion semiconductors surface processes, chemisorption, and catalysis abrasion and erosion photon-assisted surface reactions and photoelectrochemistry and interface characterization methods. The actual or potential solar applications, research issues and/or concerns, and needs and opportunities are presented in the proceedings of a recent Workshop (4) and summarized in a recent review (3). 

In sulfuric acid production, acid brick lining of membrane coated mild steel tanks and reaction vessels is considered the most durable and versatile construction material for the sulfuric acid plant. Such linings wiil reduce the steel shell temperature and prevent erosion of the normally protective iron sulfate film that forms in stagnant, concentrated (oxidizing) sulfuric acid. Dilute (red uC ing) sulfuric acid solutions are very corrosive to carbon steel, which must be protected by impermeable (e.g., elastomeric) membranes and acid brick lining systems. Such acid brick linings often employ membranes comprising a thin film of Teflon or Kynar sandwiched between layers of asphalt mastic. 

Under certain corrosive conditions, many metals form covering layers. If these are sufficiently dense, they act as protective films against the corrosive removal of the material. An example of this is the protective layer of iron oxide formed in unalloyed or low-alloy boiler tubes. Corrosion with erosion is understood as the combined action of mechanical surface removal and corrosion. With some soft and loose layers, the shear forces obtained with pure flowing liqnids at medium flow velocities are sufficient to damage the protective layer without the involvement of abrasive solid particles. Where drop impingement or cavitation is involved, the mechanical removal of material is understandable. 

Erosion damage by solid particle impact or cavitation bubble collapse to an oxide or passive film will reveal the underlying nascent surface inducing a higher activity (higher corrosion... 

However, if the flow of liquid becomes turbulent, the random liquid motion impinges on the surface to remove this protective film. Additional oxidation then occurs by reaction with the liquid. This alternate oxidation and removal of the film will accelerate the rate of corrosion. The resulting erosive attack may be uniform, but quite often produces pitted areas over the surface that can result in full perforation (Fig. 6.40). 

Sol-gel processes can be used to form nanostmctured films (typically 200 nm to 10 pm in overall thickness) that are more resistant than metals to oxidation, corrosion erosion and wear. 

---