Decoding Corrosion To Build Industry Resilience

<p style="text-align: justify;">Metal corrosion poses an interesting paradox; understanding these manifestations is crucial for implementing effective corrosion prevention strategies!</p><p style="text-align: justify;">The corrosion control business is large, and companies must understand the advantages of implementing and using anticorrosion coating systems.&nbsp;</p><p style="text-align: justify;">Corrosion is an electrochemical process that involves anodic and cathodic reactions facilitated by an electrolyte or electronic conductor. For example, buried iron pipes corrode due to the interaction with soil moisture, forming an electrochemical cell. The corrosion of buried pipe will vary directly with variations in the soil pH in which the pipe is buried. If you remove any of the above three elements, the corrosion can be aborted.</p><p style="text-align: justify;">One can understand the parameters influencing corrosion, such as soil pH affecting iron corrosion rates, by examining half-cell reactions. Even within a single metal body, localized differences in ion concentrations create anodic and cathodic areas, driving electron flow and corrosion.</p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 14pt;">Common Types of Corrosion Cells</span></h2><p style="text-align: justify;">Common corrosion cell types include:</p><p style="text-align: justify;">Metal Ion Concentration Cells.</p><p style="text-align: justify;">Oxygen Concentration Cells.</p><p style="text-align: justify;">Active-Passive Cells.</p><p style="text-align: justify;">And each of them is influenced by environmental factors like oxygen availability or salt deposits.</p><p style="text-align: justify;"><strong>Metal Ion Concentration Cells</strong></p><p style="text-align: justify;">In the presence of water, a high concentration of metal ions will exist under layered metallic surfaces, and a low concentration of metal ions will exist adjacent to the crevice created by the layered surfaces.</p><p style="text-align: justify;">The ionic gradient creates an electrical potential between the two points. The cathode (area of the metal in contact with the low concentration of metal ions) will be protected, and the anode (area of metal in contact with the high metal ion concentration) will be corroded.</p><p style="text-align: justify;"><strong>Oxygen Concentration Cells</strong></p><p style="text-align: justify;">Water normally contains dissolved oxygen. An oxygen cell can develop at any point where ambient oxygen is prevented from diffusing uniformly into the aqueous solution, thereby creating an oxygen concentration gradient between two points.</p><p style="text-align: justify;">Oxygen concentration cells are typically found under metallic or non-metallic deposits (dirt) on metal surfaces and under layered surfaces such as riveted lap joints. Oxygen cells can also develop under gaskets, wood, rubber, plastic tape, and other materials in contact with the metal surface. Corrosion will occur in the area of low oxygen concentration (anode).</p><p style="text-align: justify;"><strong>Active-Passive Cells</strong></p><p style="text-align: justify;">Active-passive cells can corrode metals that depend on a tightly adhering passivating film (usually an oxide) for corrosion protection. The corrosive action usually starts as an oxygen-concentration cell. Salt deposits on the metal surface in the presence of water containing oxygen can create the oxygen cell.</p><p style="text-align: justify;">If the passive film is flawed during application-creating holidays or breached during use beneath the salt deposit, the active metal beneath the film will be exposed to corrosive attack. An electrical potential will develop between the large area of the cathode (passive film) and the small area of the anode (active metal).</p><p style="text-align: justify;">In the case of passivating oxide films, rapid pitting of the active metal will result in the blistering of the protective film if it's flexible.</p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 14pt;">Manifestations of Corrosion</span></h2><p style="text-align: justify;"><strong>Corrosion Fatigue (Stress Corrosion Cracking)</strong></p><p style="text-align: justify;">Corrosion fatigue or Stress Corrosion Cracking (SCC) is a special case of stress corrosion caused by the simultaneous effects of tensile stress (i.e., applied loads, residual manufacturing stresses, or a combination of both) and a corrosive environment. No metal is immune from some loss of its resistance to cyclic stress if the metal is in a corrosive environment. Damage from corrosion fatigue is greater than caused by cyclic stresses and corrosive environments independently.</p><p style="text-align: justify;"><strong>Crevice Corrosion (Contact Corrosion)</strong></p><p style="text-align: justify;">Crevice corrosion is produced at the contact point of metals with metals or metals with nonmetals. It may occur at washers, under barnacles, at sand grains, under applied protective films, and at pockets formed by threaded joints.</p><p style="text-align: justify;">Whether or not stainless steels are free of pit nuclei, they are always susceptible to this kind of corrosion because a nucleus is not necessary.</p><p style="text-align: justify;"><strong>De-alloying</strong></p><p style="text-align: justify;">De-alloying is a rare form of corrosion found in alloys such as copper alloys, gray cast iron, and other alloys. De-alloying occurs when the alloy loses the active component of the metal and retains the more corrosion-resistant component in a porous "sponge" on the metal surface.</p><p style="text-align: justify;">It can also occur by re-deposition of the noble component of the alloy on the metal surface.</p><p style="text-align: justify;"><strong>Exfoliation Corrosion</strong></p><p style="text-align: justify;">Exfoliation, a form of intergranular corrosion, manifests from the forced displacement of surface metal grains by expanding corrosion products occurring at the grain boundaries just below the surface. It is visible evidence of intergranular corrosion and is most often seen on extruded sections where grain thickness is less than in rolled forms. This corrosion is common in aluminum and may occur in carbon steel.</p><p style="text-align: justify;">Concrete is a widely used structural material frequently reinforced with carbon steel reinforcing rods (rebar), post-tensioning cables, or pre-stressing wires. The steel is necessary to maintain the strength of the cement structure.</p><p style="text-align: justify;">Modern concrete formulations contain a large amount of fly ash, allowing water and salt (chloride ion) to permeate the cement structure. The chloride corrodes the embedded reinforcing metal, and as the metal corrodes, the exfoliant corrosion expands and builds pressure that eventually bursts the cement matrix.</p><p style="text-align: justify;"><strong>Filiform Corrosion</strong></p><p style="text-align: justify;">This type of corrosion occurs under painted or plated surfaces when moisture permeates through coating holidays, imperfections, or breaches. Such breaches often occur from airborne dirt trapped in protective coatings.</p><p style="text-align: justify;">Such contaminants channel water through the protective coating film. Lacquers and "quick-dry" paints are most susceptible to the problem due to their propensity to form thin films. Their use should be avoided unless field experience has proven the absence of an adverse effect.</p><p style="text-align: justify;"><strong>Fretting Corrosion</strong></p><p style="text-align: justify;">When subjected to slight vibratory motions, rapid corrosion occurs at the juncture between highly loaded metal surfaces, which is known as fretting corrosion. This type of corrosion is most common in machinery bearing surfaces, such as connecting rods, splined shafts, and bearing supports, and often causes fatigue failure. It can occur in structural members such as trusses where highly loaded bolts are used, and some relative motion occurs between the bolted members.</p><p style="text-align: justify;"><strong>Galvanic Corrosion</strong></p><p style="text-align: justify;">Galvanic corrosion is an electrochemical reaction between two dissimilar metals in the presence of an electrolyte and an electron-conductive path. This form of corrosion is characterized by aggravated corrosion at the joint between the dissimilar metals. For example, galvanic corrosion can occur at the juncture when aluminum or magnesium alloys are bolted with steel (carbon or stainless steel).</p><p style="text-align: justify;">Inherent differences in metal potentials produce galvanic corrosion. If electrical contact is made between either of these materials in the presence of an electrolyte, a galvanic cell (battery) is formed, and current will flow between them. The farther apart the metals are in the galvanic series, the greater the galvanic corrosion effect or rate will be. The more active metal is the anode, which will corrode, while the cathode will not corrode.</p><p style="text-align: justify;">Cathodic protection can also be used to control galvanic corrosion effects. This incorporates a sacrificial cathode, which intentionally corrodes to protect the anode.</p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;">&nbsp;</p>
KR Expert - Andrew Sokol

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