Shallow pit corrosion

In principle, an improvement of rust corrosion rates in low-alloyed steels is possible as well in the tidal and immersion zone, but expectations should not be too high. The significant improvements frequently cited in the literature and patent registrations are usually based on tests in the laboratory and cannot be confirmed in practical applications or only in reduced form. It must also be noted that in the low-alloyed steels shallow pit corrosion or pitting corrosion is frequently observed in addition to uniform surface corrosion. Examples can be cited in which certain alloying elements reduce the uniform surface corrosion, only to cause increased pitting at the same time. 

The steel XlOCrAl (1.4713) with about 7% Cr shows, under exposure in the immersion zone and in the tidal zone, pronounced rusting, shallow pit corrosion and crevice corrosion under bolt washers. The cathodic protection effect of iron or zinc anodes is also unsatisfactory [99]. 

The corrosion rates of these materials in almost neutral waste waters are chiefly determined by the concentration of oxygen and by its transport to the surface of the material. However, in addition to uniform surface attack, which must be taken into account by increasing the wall thickness, there may be increased local attack in the form of shallow pit corrosion and pitting corrosion due to the formation of aeration cells. The use of unalloyed and low-alloy steels as well as cast iron and cast steel is generally not recommend if there are no additional corrosion protection measures, e.g. with coatings, linings, or cathodic protection. 

Shallow pit formation is a special form of uniform corrosion, with a localized different corrosion rate, mainly observed with unalloyed and low-alloy steel (Fig. l-9f). The diameter of the shallow pit is substantially greater than its depth. Shallow-pit corrosion occurs under conditions leading to the formations of concentration cells or differential aeration cells. 

An example of shallow-pit corrosion is given in Fig. 1-34. This corrosion damage was observed in an open recirculation cooling system after 18 months of operation. The tube, made of structural steel (St 37), showed several local attacks by pitting with a local corrosion rate of up to 3 mm year" . Iron sulfide was detected in the corrosion products. This indicates the presence of anaerobic sulfate-reducing bacteria (SRB). Under strictly anaerobic conditions the bacteria reduced the sulfates to sulfides leading to enhanced corrosivity of the medium (Weber and Knopf, 1994). 

Pitting corrosion is not a major problem with copper, although it can be susceptible to shallow pitting corrosion in fresh water (Edwards et al., 1994 Taxdn, 1996) and sea water, especially under stagnant conditions. Copper, especially in the cold-worked condition, is susceptible to stress-corrosion cracking in the presence of ammonia and nitrite solutions (Yu and Parkins, 1987). 

Types of damage can be classified as uniform or localized metal removal, corrosion cracking or detrimental effects to the environment from the corrosion products. Local attack can take the form of shallow pits, pitting, selective dissolution of small microstructure regions of the material or cracking. Detrimental effects are certainly not the case with buried pipelines, but have to be considered for environments in vessels and containers. It is usual, where different results of reactions lead... 

Sulphur dioxide in the air originates from the combustion of fuel and influences rusting in a number of ways. For example, Russian workers consider that it acts as a cathodic depolariser , which is far more effective than dissolved oxygen in stimulating the corrosion rate. However, it is the series of anodic reactions culminating in the formation of ferrous sulphate that are generally considered to be of particular importance. Sulphur dioxide in the air is oxidised to sulphur trioxide, which reacts with moisture to form sulphuric acid, and this in turn reacts with the steel to form ferrous sulphate. Examination of rust Aims formed in industrial atmospheres have shown that 5% or more of the rust is present in the form of iron sulphates and FeS04 4H2 0 has been identified in shallow pits . 

Corrosion described as pitting is usually localized and can be in the form of small, deep pits as well as large, shallow pits. It can occur in areas of stagnation where an anode site can develop. 

It is concluded that the pin failed due to fatigue initiated at the outside cylindrical surface where wear and pitting corrosion occurred. The fracture initiated at a shallow circumferential groove and corrosion of the fracture occurred after the rupture. There was no evidence of stress-corrosion cracking. 

If corrosion products are formed at the metal surface, the corrosion process can easily become nonuniform and shallow pit formation can take place, for example. 

Standards define uniform surface corrosion as corrosion practically equal to mass loss over the entire surface (homogeneous mixed electrode), and nonuniform corrosion (shallow pit formation) as corrosion with locally different mass losses. 

In the case of nonuniform corrosion, shallow pit formation, the real corrosion current density and mean corrosion current density ig are not equal. Accordingly, nonuniform corrosion is characterized by a locally differing corrosion rate. The conditions for this may occur, for example, during the course of corrosion through a nonhomogeneous covering of the reaction product on the material. 

The floors were attacked by microorganisms. Shallow pits were formed on the floor after 25 years of service. Localized corrosion rates reached up to 0.5 mm/year. Heterotrophic bacteria and sulfate-reducing bacteria (SRB) were found in the sludge. 

Erosion corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in subsequent corrosion rate increases via either electrochemical or chemical processes. It is often accepted that a critical fluid velocity must be exceeded for a given material. The mechanical damage by the impacting fluid imposes disruptive shear stresses or pressure variations on the material surface and/or the protective surface film. Erosion corrosion may be enhanced by particles (solids or gas bubbles) and impacted by multi-phase flows [29]. Increased flow stream velocities and increases of particle size, sharpness, density, and concentration increase the erosion corrosion rate. Increases in fluid viscosity, density, target material hardness, and/or pipe diameter tend to decrease the corrosion rate. The morphology of surfaces affected by erosion corrosion may be in the form of shallow pits or horseshoes or other local phenomena related to the flow direction. 

The shiny metal appearance will gradually disappear and the surface will roughen under the formation of a gray patina of corrosion products. If soot is present in the atmosphere, it will be absorbed by the corrosion products and give the patina a dark color. If the surface has been anodized, the shiny metal appearance will be retained. Except in highly polluted atmospheres the low penetration rate and shallow pitting do not usually affect the mechanical properties of the aluminum. 

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