Steels continued rusting

Rusting can be prevented by painting or coating with a continuous layer of another metal which does not itself corrode rapidly, for example zinc or tin. More recently, steel has been coated with plastics by electrophonetic decomposition from an emulsion of the plastic. 

Oxygen concentration is held almost constant by water flow outside the crevice. Thus, a differential oxygen concentration cell is created. The oxygenated water allows Reaction 2.2 to continue outside the crevice. Regions outside the crevice become cathodic, and metal dissolution ceases there. Within the crevice. Reaction 2.1 continues (Fig. 2.3). Metal ions migrating out of the crevice react with the dissolved oxygen and water to form metal hydroxides (in the case of steel, rust is formed) as in Reactions 2.3 and 2.4 ... 

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

Although some sources of possible contamination are readily identified, and often may be kept from occurring, past bad disposal methods continue to plague the environment. Industries small and large have in past decades disposed of waste product by shallow burial. Liquid wastes were buried in steel containers, which after 30 or 40 years begin to rust through and leak their contents into the soil. The location of most of these burial sites are found only after environmental pollution has begun. 

Chromium plated steel (steel is mostly iron) is often used in motor car bumpers. If the bumper is scratched, will the chromium continue to protect the bumper against rusting ... 

Metals vary greatly in their corrosion-resistance - chromium and titanium have good resistance, while steel readily corrodes. The oxide film formed on chromium and titanium closely adheres to the surface and protects the metal from further oxidation. In the case of steel, the oxide film in the form of rust is loose, allows moisture to be retained, and promotes further corrosion. If corrosion is allowed to continue, the steel will eventually be completely consumed, i.e. the metal will have returned to the condition of the ore from which it was extracted. 

Rust formed on MS and WS panels was exposed in humid SO2 for 9 m and were analysed by SEM and EDX on the exposed surface and cross section of the panels (Figs. 3.62 and 3.63 and Table 3.30). Rust/steel interface of WS was found to be continuous and uniform than MS. The latter showed abmpt crystalline layo- at the interface. In upper layer on WS mst, Mn and in inner layo- Cr, Cu, Ni WCTe found. The alloying elements are wined away from the surface. However, these were intact at the interior layer and made mst more compact and impervious to corroding electrolyte to penetrate further. No traces of Cu, Cr, etc. were recorded in the upper layer of this rust... 

Rust formed in continuous humidity on weathering steel was not stable and protective confirmed by Raman spectroscopy. Weathering steels developed passive layers in Na2S04 but mild steel did not... 

Rust formed in continuous humidity test on weathering steel was not stable and protective. Again WS developed more protective oxide than MS when exposed in humid SO2 for longer period. No damage of coating was noted with initial exposure of 18 months on coated MS and WS. The data generated with accelerated laboratory tests on both coated and uncoated MS and WS panels have wide gap and difiicult to predict the performance in field exposure. 

Thus it can be seen that corrosion (dissolution of the metal) is an electrochemical process. Rusting does, however, require an additional oxidation step. If the final corrosion product can be formed as a strongly adherent, insoluble and impermeable layer on the metal surface, corrosion will decline, but if it is loose or permeable, the corrosion process will continue. In real life, the above processes are modified by the presence of other chemical species, even in trace amounts, and on steel the layer is commonly loose and permeable. In fact, corrosion can be inhibited by one of the following techniques ... 

---