FORMS OF CORROSION Corrosion may be classified in different ways WET CORROSION DRY CORROSION Corrosion may be classified in different ways Wet / Aqueous corrosion & Dry Corrosion Room Temperature/ High Temperature Corrosion CORROSION ROOM TEMPERATURE HIGH TEMPERATURE

WET & DRY CORROSION Wet / aqueous corrosion is the major form of corrosion which occurs at or near room temperature and in the presence of water Dry / gaseous corrosion is significant mainly at high temperatures

WET / AQUEOUS CORROSION Based on the appearance of the corroded metal, wet corrosion may be classified as: Uniform or General Galvanic or Two-metal Pitting Environment-assisted cracking Intergranular Crevice Velocity-assisted Dealloying Fretting

Uniform Corrosion

Uniform Corrosion This one is common in steel that is unprotected by any surface coating. Most noticeable. Surface effect, leaving rust on the surface. The good thing about this, if there is one, is that the corrosion is widely spread around.

Uniform Corrosion Corrosion over the entire exposed surface at a uniform rate. e.g.. Atmospheric corrosion. Maximum metal loss by this form. Not dangerous, rate can be measured in the laboratory.

Uniform Corrosion EXAMPLES: 1.rusting of iron 2.tarnishing of silver 3.Fogging of nickel 4.high - temperature oxidation of metals

Corrosion Rate and Classification of Metals mm/y – millimeters penetration per year gmd – grams per square meter per day ipy – inches penetration per year mpy – mils penetration per year (1000 mil = 1 inch) mdd – milligrams per square decimeter per day

Classification of metallic materials according to their rate of uniform attack <0.005 ipy (<0.15 mm/y) : Metals in this category have good corrosion resistance and can be used for critical parts 0.005 to 0.05 ipy (0.15 mm/y to 1.5 mm/y) : Metals in this group are satisfactory if a higher rate of corrosion can be tolerated >0.05 ipy (>1.5 mm/y) : Usually not satisfactory

Galvanic Corrosion: Possibility when two dissimilar metals are electrically connected in an electrolyte* Results from a difference in oxidation potentials of metallic ions between two or more metals. The greater the difference in oxidation potential, the greater the galvanic corrosion. Refer to Galvanic Series The less noble metal will corrode (i.e. will act as the anode) and the more noble metal will not corrode (acts as cathode). Perhaps the best known of all corrosion types is galvanic corrosion, which occurs at the contact point of two metals or alloys with different electrode potentials.

Galvanic Corrosion When two dissimilar metals are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junction Fig. Al sheets covering underground Cu cables

Galvanic Series: Questions: Worst combination? Aluminum and steel? Titanium and Zinc? Stainless Steel and Copper?

Note, positions of SS and Al GALVANIC SERIES Mercury Platinum Gold Zirconium Graphite Titanium Hastelloy C Monel Stainless Steel (316-passive) Stainless Steel (304-passive) Stainless Steel (400-passive) Nickel (passive oxide) Silver Hastelloy 62Ni, 17Cr Silver solder Inconel 61Ni, 17Cr Aluminum (passive AI203) 70/30 copper-nickel 90/10 copper-nickel Bronze (copper/tin) Copper Brass (copper/zinc) Alum Bronze Admiralty Brass Nickel Naval Brass Tin Lead-tin Lead Hastelloy A Stainless Steel (active) 316 404 430 410 Lead Tin Solder Cast iron Low-carbon steel (mild steel) Manganese Uranium Aluminum Alloys Cadmium Aluminum Zinc Beryllium Magnesium PASSIVE: will not corrode – act as cathode. These elements are least likely to give up electrons! Note, positions of SS and Al ACTIVE: will corrode – act as anode. These elements most likely to give up electrons!

Big Cathode, Small Anode = Big Trouble

Liquid Cell Battery: dry cell is a galvanic electrochemical cell with a pasty low-moisture electrolyte. A wet cell, on the other hand, is a cell with a liquid electrolyte, such as the lead-acid batteries in most cars.

Dry Cell - Zinc-carbon battery - oxidation reaction that happens at zinc = anode : Zn(s) → Zn2+(aq) + 2 e- - reduction reaction at carbon rod = cathode

Galvanic Corrosion Galvanic corrosion (also called “dissimilar metal corrosion”) refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. In a bimetallic couple, the less noble material becomes the anode and tends to corrode at an accelerated rate, compared with the uncoupled condition and the more noble material will act as the cathode in the corrosion cell.

Bilge pump - Magnesium shell cast around a steel core. Galvanic Corosion Dissimilar metals are physically joined in the presence of an electrolyte. The more anodic metal corrodes. c16f14 Bilge pump - Magnesium shell cast around a steel core.

Dissimilar metals, the damage occurs at the anode. Galvanic Corrosion Steel screws and brass Steel screw in Mg Dissimilar metals, the damage occurs at the anode.

Design for Galvanic Corrosion? Material Selection: Do not connect dissimilar metals! Or if you can’t avoid it: Try to electrically isolate one from the other (rubber gasket). Make the anode large and the cathode small Bad situation: Steel siding with aluminum fasteners Better: Aluminum siding with steel fasteners Eliminate electrolyte Galvanic of anodic protection

Design for Galvanic Corrosion? Galvanic severity depends on: NOT Not amount of contact Not volume Not mass Amount of separation in the galvanic series Relative surface areas of the two. Severe corrosion if anode area (area eaten away) is smaller than the cathode area. Example: dry cell battery

Steel bolt (less noble) is isolated from copper plates.

Highly localized. Goes deep into the metal. Pitting It is based on low oxygen concentration at the bottom of the pit. This is very common in materials that protect themselves with a passive layer, i.e. stainless steel and aluminum. Highly localized. Goes deep into the metal.

Pitting A form of extremely localized attack causing holes in the metal Most destructive form Autocatalytic nature Difficult to detect and measure Mechanism

304 stainless steel / acid chloride solution Pitting Pitting is a localized form of corrosive attack.  Pitting corrosion is typified by the formation of holes or pits on the metal surface.  Pitting can cause failure, yet the total corrosion, as measured by weight loss, may be minimal. c16f17 304 stainless steel / acid chloride solution 5th Century sword Boiler tube

Pitting If appreciable attack is confined to a relatively small, fixed area of metal, acting as anode, the resultant pits are described as deep. If the area of attack is relatively larger and not so deep, the pits are called shallow. Depth of pitting is sometimes expressed by the pitting factor , the ratio of deepest metal penetration to average metal penetration as determined by the weight loss of the specimen. A pitting factor of unity represents uniform attack. Iron buried in the soil corrodes with formation of shallow pits, whereas stainless steels immersed in seawater characteristically corrode with formation of deep pits.

Intergranular Corrosion Again, stainless steel is the ideal victim here. The problem is triggered by improper heating, and often this comes with welding. Carbides of chromium form in the grain boundary regions. The chromium is tied up in the carbides. It can’t protect by forming the passive layer. PLUS, there is a dissimilarity in metals producing a small but definite galvanic corrosion. This is a localized type of attack at the grain boundaries of a metal, resulting in loss of strength and ductility. Grain – boundary material of limited area, acting as anode, is in contact with large areas of grain acting as cathode. The attack is often rapid, penetrating deeply into the metal and sometimes causing catastrophic failures. Improperly heat - treated stainless steels or Duralumin - type alloys (4% Cu – Al) are among the alloys subject to intergranular corrosion. At elevated temperatures, intergranular corrosion can occur because, under some conditions, phases of low melting point form and penetrate along grain boundaries; for example, when nickel - base alloys are exposed to sulfur - bearing gaseous environments, nickel sulfide can form and cause catastrophic failures. This type of attack is usually called sulfidation .

Intergranular Corrosion Corrosion which occurs preferentially at grain boundries. Why at grain boundries? Higher energy areas which may be more anodic than the grains.

Intergranular Corrosion The grain boundaries in metals are more active than the grains because of segregation of impurities and depletion of protective elements. So preferential attack along grain boundaries occurs. e.g. weld decay in stainless steels

Intergranular Corrosion How to recognize it? Near surface Corrosion only at grain boundries Corrosion normally at uniform depth for all grains.

Example 1: Intergranular Corrosion Sensitization of stainless steels: Heating up of austenitic stainless steel causes chromuim carbide to form in the grains. Chromuim is therefore depleted near the grain boundries causing the material in this area to essentially act like a low-alloy steel which is anodic to the chromium rich grains. Preferential Intergranular Corrosion will occur parallel to the grain boundary – eventually grain boundary will simply fall out!!

Intergranular corrosion Corrosion along grain boundaries, often where precipitate particles form. c16f18

Intergranular Corrosion

Example2:Intergranular Corrosion Exfoliation corrosion in Aluminum that has been heavily worked, such as in extrusion. Corrosion products start to build up in between the long elongated grains, separating them and lead in to increased corrosion propagation through the metal.

Intergranular Corrosion

Design for Intergranular corrosion Watch welding of stainless steels (causes sensitization). Always needs proper annealling after welding to redistribute Cr. Use low carbon grade stainless to eliminate sensitization (304L or 316L). Add alloy stabilizers like titanium which ties up the carbon atoms and prevents chromium depletion.

CREVICE CORROSION Intensive localized corrosion within crevices & shielded areas on metal surfaces Small volumes of stagnant corrosive caused by holes, gaskets, surface deposits, lap joints

Narrow and confined spaces. Crevice Corrosion c16f15 Narrow and confined spaces.

Crevice Corrosion

Crevice Corrosion This is a concentration cell in action. Notice how the damage occurs in out of sight places.

VELOCITY ASSISTED CORROSION Fast moving corrosives cause: a) Erosion-Corrosion b) Cavitation damage c) Impingement attack

Erosion Corrosion This is caused by the impingement of a high velocity turbulent flow on a surface. The flow is often multi-phase. This means there can be entrained solid particles, or even gas bubbles, as in cavitation of a propeller. The flow will carry away any protective layer that was intended to protect the material, and even abrade the flow surface.

Erosion-corrosion c16f20 Combined chemical attack and mechanical wear (e.g., pipe elbows) c16f20 Brass water pump

Cavitation Damage Cavitation is a special case of Erosion-corrosion. In high velocity systems, local pressure reductions create water vapour bubbles which get attached to the metal surface and burst at increased pressure, causing metal damage.

Cavitation Erosion Cavitation – erosion is the loss of material caused by exposure to cavitation, which is the formation and collapse of vapor bubbles at a dynamic metal – liquid interface — for example, in rotors of pumps or on trailing faces of propellers. This type of corrosion causes a sequence of pits.

Impingement Attack when subjected to high - velocity liquids, undergo a pitting type of corrosion called impingement attack. Copper and brass condenser tubes, for example, are subject to this type of attack.

Dealloying: When one element in an alloy is anodic to the other element. Example: Removal of zinc from brass (called dezincification) leaves spongy, weak brass. Brass alloy of zinc and copper, and zinc is anodic to copper (see galvanic series).

Dealloying Two common types: Dezincification: preferential removal of zinc in brass Try to limit Zinc to 15% or less and add 1% tin. Cathodic protection Graphitization : preferential removal of Fe in Cast Iron leaving graphite (C).

Dealloying Dealloying is the selective removal of an element from an alloy by corrosion. One form of dealloying, dezincification, is a type of attack occurring with zinc alloys (e.g., yellow brass) in which zinc corrodes preferentially, leaving a porous residue of copper and corrosion products. The alloy so corroded often retains its original shape, and may appear undamaged except for surface tarnish, but its tensile strength and ductility are seriously reduced. Dezincified brass pipe may retain sufficient strength to resist internal water pressures until an attempt is made to uncouple the pipe, or a water hammer occurs, causing the pipe to split open. Parting is similar to dezincification in that one or more reactive components of the alloy corrode preferentially, leaving a porous residue that may retain the original shape of the alloy. Parting is usually restricted to such noble metal alloys as gold – copper or gold – silver and is used in gold refining. For example, an alloy of Au – Ag containing more than 65% gold resists concentrated nitric acid as well as does gold itself. However, on addition of silver to form an alloy of approximately 25% Au – 75% Ag, reaction with concentrated HNO3 forms silver nitrate and a porous residue or powder of pure gold. Copper - base alloys that contain aluminum are subject to a form of corrosion resembling dezincification, with aluminum corroding preferentially.

Dealloying c16f21 Preferred corrosion of one element/constituent [e.g., Zn from brass (Cu-Zn)]. Dezincification.

Dealloying Alloys exposed to corrosives experience selective leaching out of the more active constituent. e.g. Dezincification of brass. Loss of structural stability and mechanical strength

Dealloying: Danger! The alloy may not appear damaged May be no dimensional variations Material generally becomes weak – hidden to inspection!

Fretting Corrosion Fretting corrosion , which results from slight relative motion (as in vibration) of two substances in contact, one or both being metals, usually leads to a series of pits at the metal interface. Metal - oxide debris usually fills the pits so that only after the corrosion products are removed do the pits become visible.