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Forms of Corrosion in the Petrochemical Industry

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Forms of Corrosion in the Petrochemical Industry Corrosion in the Petrochemical Industry (06393G) Copyright © 1994 ASM International® Linda Gaverick, editor All Rights Reserved. www.asminternational.org Forms of Corrosion in the Petrochemical Industry OVER THE YEARS, corrosion scientists and system, these categories are not distinct or all-in­ Many authors classify atmospheric corro­ engineers have recognized that corrosion mani­ clusive and do not necessarily represent the only sion under categories of dry, damp, and wet, thus fests itself in forms that have certain similarities mode of attack that may be observed. emphasizing the different mechanisms of attack and therefore can be categorized into specific under increasing humidity or moisture. groups. However, many of these forms are not unique but involve mechanisms that have over­ General Corrosion Types of Atmospheric Corrosion lapping characteristics that may influence or control initiation or propagation of a specific Dry Corrosion. In the absence of moisture, type of corrosion. General corrosion, as described in this article, most metals corrode very slowly at ambient tem­ The most familiar and often used categoriza­ refers to corrosion dominated by uniform thin­ peratures. Accelerated corrosion under dry con­ tion of corrosion is probably the eight forms pre­ ning that proceeds without appreciable localized ditions at elevated temperatures is covered in the sented by Fontana and Greene (Ref 1): uniform attack. Weathering steels and copper alloys are section "High-Temperature Corrosion" in this attack, crevice corrosion, pitting, intergranular good examples of materials that typically exhibit article. Dry corrosion at ambient temperature oc­ corrosion, selective leaching, erosion corrosion, general attack, while passive materials, such as curs on metals that have a negative free energy of stress corrosion, and hydrogen damage. This stainless steels or nickel-chromium alloys, are oxide formation and thus form a rapid thermody­ classification of corrosion was based on visual generally subject to localized attack. Under spe­ namically stable film in the presence of oxygen. characteristics of the morphology of attack. Fon­ cific conditions, however, each material may TYpically, these films are desirable because they tana and Greene's introductory remarks in their vary from its normal mode of corrosion. Exam­ are defect free, nonporous, and self-healing and chapter on forms of corrosion indicate that this ples describing the environmental conditions act as a protective barrier to further corrosive at­ classification is arbitrary and that many of the that promote uniform attack will be discussed tack of the base metaL Metals such as stainless forms are interrelated, making exact distinction throughout this article. steels, titanium, and chromium develop this type impossible. Other prominent corrosion authors In this section, the four specific types of gen­ of protective film. Porous and nonadhering films such as Uhlig (Ref 2) and Evans (Ref 3) have eral corrosion most relevant to the petrochemical that form spontaneously on nonpassive metals as avoided a classification format and have simply industry are discussed. Atmospheric corrosion is unalloyed steel are normally not desirable. discussed the classical types of corrosion (for ex­ probably the most common form of corrosion Tarnishing of copper and silver in dry air with ample, pitting and crevice corrosion) as they re­ and may well be the most costly. Galvanic corro­ traces of hydrogen sulfide (H2S) is an example of late to specific metals and alloys. sion is an electrochemical form of corrosion that a nondesirable film formation at ambient tem­ Substantial advances in the field of corrosion protects cathodic areas at the expense of anodic peratures caused by lattice diffusion. For tarnish­ science have begun to define the mechanisms of areas. Stray-current corrosion is similar to gal­ ing to occur, sulfur impurities must be present. many forms of corrosion more clearly. However, vanic corrosion, but does not rely on electro­ The sulfides increase the likelihood of defects in rather than placing the mechanisms into distinct chemically induced driving forces to cause rapid the oxide-lattice and thus destroy the protective categories, the overlap between many of the attack. High-temperature (gaseous) corrosion is nature of the natural film, which leads to a tar­ forms has become greater. For example, there is an area of great concern, particularly for the in­ nished surface. Surface moisture is not necessary evidence that hydrogen may dominate the crack dustrial sector. for tarnishing to occur, and in some cases, such as initiation or crack propagation portion of fracture As noted in the introduction, some of thecate­ copper in the presence of trace amounts of H2S, in some metal/solution systems where stress-cor­ gories of general corrosion described in this arti­ moisture can actually retard the process of tar­ rosion cracking occurs. Additionally, in some cle also manifest themselves as other forms of nishing. In general, dry corrosion plays an insig­ metal systems where dealloying (selective leach­ corrosive attack, such as stress-corrosion crack­ nificant part in atmospheric corrosion as a whole. ing) occurs, this form of corrosion may be a pre­ ing, dealloying, or pitting. However, because Damp corrosion requires moisture in the at­ cursor to stress-corrosion cracking. uniform thinning plays an important role in all of mosphere and increases in aggressiveness with the In a similar vein, the magnitude of contribu­ the categories described, each can and will be moisture content. When the humidity exceeds a tion of stress or corrosion to stress-corrosion discussed under general corrosion. critical value, which is around 70% relative humid­ cracking, hydrogen damage, or liquid metal em­ ity, an invisible thin film of moisture will form on brittlement is not currently understood and can Atmospheric Corrosion the surface of the metal, providing an electrolyte for affect whether just pitting or crevice attack oc­ current transfer. The critical value depends on sur­ curs or environmental cracking results. The tran­ Atmospheric corrosion is defined as the face conditions such as cleanliness, corrosion prod­ sition from uniform corrosion to highly localized corrosion or degradation of material exposed uct buildup, or the presence of salts or other attack is not clearly understood, and there are to the air and its pollutants rather than im­ contaminants that are hygroscopic and can absorb conditions where a distinction cannot be drawn. mersed in a liquid. This has been identified as water at lower relative humidities. The forms of corrosion presented in this article one of the oldest forms of corrosion and has Wet corrosion occurs when water pockets or were categorized to represent the mechanisms of at­ been reported to account for more failures in visible water layers are formed on the metal sur­ tack involved rather than to emphasize the visual terms of cost and tonnage than any other single faces because of sea spray, rain, or drops of dew. characteristics. However, as with any classification environment. Crevices or condensation traps also promote the 4 I Corrosion in the Petrochemical Industry Fig. 3 Corroded weathering steel gutter. Courtesy of R.H. Heidersbach, California Polytechnic State University Fig. 1 Corroded steel formwork on the ceiling of a park- Ing garage. The seams 1n th1s corru gated structure ac t as condensation traps and lead to wet atmospheric cor- rosion. Courtesy of R.H . Heidersbach, California Polytec h- •• nic State University 1~1 Fig 5 Corroded regions on a painted highway bridge. • Courtesy of R.H. Heidersba ch, California Poly- tec hnic State Univers ity Fig 2 Corroded weathering steell-beam. Note how cor- • rosion has thinned the bottom of the vertical web where corrosion prod ucts have fallen and formed a moist corros ive deposit. Courtesy of R.H. Heidersbach, Ca lifornia Polytec hnic State University pooling of water and lead to wet atmospheric cor­ rosion even when the flat surfaces of a metal Fig 6 Delamination of plain carbon steel due to 502 + component appear to be dry (Fig. I). • ash deposit outside the boiler area of a coal-fired During wet corrosion, the solubility of corro­ Fig. 4 Corroded weath ering steel highway bridge girder. power plant. Courtesy of D.M . Berger, Gilbert/Common­ Courtesy of D. M annmg, Ontano Mm1stry of High­ wealth sion product can affect the corrosion rate. Typi­ ways and Communications cally, when the corrosion product is soluble, the corrosion rate will increase. This occurs because Small additions of copper (0.1 %) will in­ the dissolved ions normally increase the conduc­ ronment. For example, steel pillars 25 m (80 ft) crease the resistance of steel to a sulfur polluted tivity of the electrolyte and thus decrease the in­ from the seacoast will corrode 12 times faster environment by enhancing the formation of a ternal resistance to current flow, which will lead than the same steel pillars 250m (800ft) further tighter, more protective rust film . Additions of to an increased corrosion rate. Under alternating inland. The level of marine salts found at the two nickel and chromium will accomplish the same wet and dry conditions, the formation of an insol­ locations can explain the difference in the ob­ end. Nickel and copper alloys form insoluble sul­ uble corrosion product on the surface may in­ served corrosion rates. More detailed informa­ fates that help to protect the base metal and are crease the corrosion rate during the dry cycle by tion on marine atmospheres and their effect on therefore used extensively in industrial environ­ absorbing moisture and continually wetting the the corrosivity of metals and alloys can be found ments. The remarkable longevity of ancient iron surface of the metal. in the article "Marine Corrosion" in Volume 13 is probably due to a SOrfree atmosphere rather The rusting of iron and steel and the forma­ oftheASM Handbook. than a high degree of resistance to general corro­ tion of patina on copper are examples of metals Industrial atmospheres are more corrosive than sive attack.
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