Carbon steels crevice corrosion

Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heat-affected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design. 

Weld attack. Welds are often more susceptible to corrosion than other areas (see Chap. 15, Welds Defects ). Welds may contain porosity, crevices, high residual stresses, and other imperfections that favor attack. Carbon steel welds are usually ditched by acid attack (Fig. 7.10). 

Access of air and water will also affect the corrosion rate. Metal inserts in corrosive plastics are most actively attacked at the plastic/metal/air interfaces with certain metals, notably aluminium titaniumand stainless steel, crevice effects (oxygen shielding and entrapment of water) frequently accelerate attack. Acceleration of corrosion by bimetallic couples between carbon-fibre-reinforced plastics and metals presents a problem in the use of these composites. 

In the case of carbon steel, the accumulation of Fe2+ in the crevice attracts chloride ions (Cl-) and aggressive pitting corrosion may occur. 

For crevices such as in those in socket welds, the metal in the crevice is likely to be anodic. Crevice corrosion and under-deposit corrosion can be serious problems in oxide-stabilized materials such as aluminum and the stainless steels. Crevices and deposits can also accelerate corrosion in metals (such as carbon steel) that do not exhibit both active and passive states. However, the rate of corrosion is much slower in such materials because they lack the galvanic driving force of the active-passive states characteristic of the oxide-stabilized metals and alloys. The anode areas in crevices and under deposits are typically smaller than the cathode areas. This difference accelerates the corrosion rate. 

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

Carbon steels are susceptible to stress corrosion cracking in alkaline environments at elevated temperature. At the start of the industrial era, many steel-riveted boilers burst due to SCC because the water treatment used permitted the establishment of alkaline conditions in crevices underneath the rivets. The phenomenon was referred to as caustic embrittlement. 

As a general rule corrosion rate increases with temperature. One notable exception is the corrosion of carbon steel by water in an open container (Fig. 2) that was discussed earlier. Temperature also inQuences the onset of localized corrosion such as pitting and crevice corrosion of passive alloys. 

If carbon or low alloy steel cannot be safely used, corrosion resistant alloys must be evaluated. The production environment must be dupHcated in alloy corrosion tests in the same manner as for carbon or low alloy steels. Standard corrosion tests modified to use the production environment are conducted for the corrosion resistant alloys with due consideration given the probable failure modes, e.g., crevice and pitting corrosion for stainless steels. On completion of this test sequence, the corrosion resistant alloy or alloys to use for tubulars, wellheads, and facilities construction are specified. 

An acceptable life for undergound pipelines can sometimes be realized through the use of corrosion-resistant piping. Copper, aluminum, and stainless steel piping are sometimes used for this purpose. All three alloys can have greater corrosion resistance than carbon steel or cast iron. However, they are not immune to corrosion and are often more susceptible to localized corrosion such as pitting, crevice corrosion, and SCC than carbon steel or cast iron. Further, corrosion protection in the form of coatings and cathodic protection are frequently used. 

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