Uniform corrosion examples

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

Almost all new metallic surfaces exposed to the environment are sooner or later coated with a layer of corrosion products metal oxides, sulfides, and carbonates, for example, are common corrosion products formed when a metal or alloy interacts with contaminants in the environment. If the layer is continuous and stable, as in uniform corrosion, it may conceal the underlying metal from further exposure and protect it from additional corrosion if it is discontinuous, or chemically unstable, however, the metal surface below the initial layer of corrosion products remains in contact with the environment. Exposed to humidity and pollutants, the corrosion process continues, penetrating deeper into the metallic bulk and eventually resulting in its total destruction. 

Dissolution of steel or zinc in sulfuric or hydrochloric acid is a typical example of uniform electrochemical attack. Steel and copper alloys are more vulnerable to general corrosion than other alloys. Uniform corrosion often results from atmospheric exposure (polluted industrial environments) exposure in fresh, brackish, and salt waters or exposure in soils and chemicals. The rusting of steel, the green patina on copper, tarnishing silver and white mst on zinc on atmospheric exposure are due to uniform corrosion.14... 

Protective passive films similar to that of stainless steel, for example, result in uniform corrosion because of the mobility of active sites that passivate readily. Corrosion products and/or passive films are characteristic of numerous electrochemical corrosion of alloys. A film is protective depending on coverage capacity, conductivity,... 

Uniform corrosion usually occurs in fairly aggressive environments that attack the whole surface. Examples include carbon steel in seawater or acids, or aluminum alloys in strong alkali. The rate of metal loss is usually rather high, but, because it is distributed over the whole surface, the performance can usually be predicted, and managed with corrosion allowances, in most situations. Thus, sheet steel piling is often used in seawater without any corrosion protection, the corrosion rate of around 0.1 mm/yr, coupled with the relatively thick steel sections, giving an acceptable life. 

Prevention—Hydrogen embrittlement is prevented or minimized by (1) reducing the uniform corrosion rate on a metal surface (by coating/painting the metal surface, for example), to decrease the rate of hydrogen evolution on the metal surface (2) baking the metal (hydrogen evolution is an... 

Types of Corrosion Phenomena. The major categories of phenomena ( include uniform, localized, and pitting corrosion selective dissolution and corrosion acting together with a mechanical phenomenon. In uniform corrosion, all areas corrode at the same rate. Examples of uniform corrosion include tarnishing and active dissolution of metals in acids. In localized corrosion some areas corrode more readily than others crevice corrosion and filiform... 

These three regimes are examples of uniform corrosion and can be contrasted with the fourth, i.e. solutions containing aggressive ions such as chloride, where protective layers are not formed, and localised, pitting corrosion takes place. 

Very often the long experience with chemicals operating as corrosion inhibitors, e. g. in the oil field, gas or petroleum industry, is taken as an example for the successful use of corrosion inhibitors for many decades. This undoubtedly is true and the overwhelming majority of literature on corrosion inhibitors deals with the effects of inhibitors on uniform corrosion, e. g. of steel in acidic or neutral solutions, where they can be classified into [2] a) adsorption inhibitors, acting specifically on the anodic or on the cathodic partial reaction of the corrosion process or on both reactions (mixed inhibitor), b) JUm-forming inhibitors, blocking the surface more or less completely, and c) passivators, favouring the passivation reaction of the steel (e. g. hydroxyl ions). 

It should be pointed out that corrosion is a system property. Small changes of the electrolyte, for example, pH, temperature, content of the oxidizing agent, or the addition of halide ions or metal ions of higher valence can change corrosion behavior drastically. Further factors influencing the rate of uniform corrosion are... 

Several examples of the results of crevice corrosion are shown in Figs. 3 to 6. The similarities in topography amongst the examples include the accelerated attack of the substrate under the crevice former and the virtual absence of attack on the fully exposed surface. The accelerated attack within the crevice usually appears as uniform corrosion or pitting. In some cases, it is thought that the attack starts as metastable pits that coalesce into a more uniform attack. 

Corrosion of the pipe wall can occur either internally or externally. Internal corrosion occurs when corrosive fluids or condensates are transported through the pipelines. Depending on the nature of corrosive liquid and the transport velocity, different forms of corrosion may occur, namely, uniform corrosion, pitting/crevice corrosion, and erosion-corrosion. Figure 3.8 shows an example of internal corrosion that occurred in a crude oil pipeline because of high levels of salt water and carbon dioxide (CO2). 

Non-uniform corrosion or local corrosion. One example is pitting corrosion, the formation of small round pits in the wall of sanitary pipes. 

Figure 10.1 Examples of different forms of corrosion are (a) uniform corrosion, (b) non-uniform corrosion, (c) inter-granular corrosion, (d) stress corrosion cracking, (e) crevice corrosion, and (f) selective corrosion.

Corrosion can affect an entire surface of a metal or just local spots. Uniform corrosion of the complete surface usually only happens in acidic conditions [2]. This generally results in overall thinning and causes no major damage. On the other hand, a very detrimental form of corrosion is pitting. This type is found at a single location on the surface and creates a pit or cavity which is difficult to prevent and often hard to detect. It can result in structural failure (example a cracked pipe). 

Researchers have identified many different forms of corrosion. The rusting of automobile bodies is an example of uniform corrosion and is one of the most visible forms of corrosion. What conditions are necessary for this process to occur Another important form of corrosion is galvanic corrosion, which occurs only when two different metals contact each other in the presence of an appropriate electrolyte. What is so special about the contact of two different metals Other forms of corrosion tend to require specific conditions, yet many of these situations... 

Figure 13.6 I The rusting of iron exposed to the weather—even support iron underneath concrete—is an example of uniform corrosion. Iron is oxidized, and oxygen from the air is reduced. Water is needed for ion mobility between the anodic and cathodic regions, and the presence of ionic salts speeds the reaction considerably.

The rate of general or uniform corrosion is expressed as weight loss per unit area and time, for example, rngdra" day (mdd), or as loss in the thickness of the sample, for example, millimeters per year (mm/y) or mils/year (mpy). These two descriptions can... 

On a laboratory scale, it has been demonstrated in recent years that thin films of silanes applied to metals can protect the metal from many forms of corrosion, including uniform corrosion, pitting corrosion, stress corrosion cracking, crevice corrosion and other forms, in addition to providing excellent and durable paint adhesion (see Durability - fundamentals). Such thin films are typically of not more than 300-mn thickness. They can be applied by immersion of the metal into a dilute silane solution, for example, 5% in water or water/alcohol mixtures, as not all silanes dissolve in water. Brushing, wiping or spraying application methods can also be used. 

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