Presentation on theme: "FORMS OF CORROSION Corrosion may be classified in different ways Wet / Aqueous corrosion & Dry Corrosion Room Temperature/ High Temperature Corrosion."— Presentation transcript:
FORMS OF CORROSION Corrosion may be classified in different ways Wet / Aqueous corrosion & Dry Corrosion Room Temperature/ High Temperature Corrosion CORROSION WET CORROSION DRY CORROSION CORROSION ROOM TEMPERATURE CORROSION HIGH TEMPERATURE CORROSION
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
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 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 A.<0.005 ipy (<0.15 mm/y) : Metals in this category have good corrosion resistance and can be used for critical parts B 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 C.>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
Questions: 1.Worst combination? 2.Aluminum and steel? 3.Titanium and Zinc? 4.Stainless Steel and Copper? Galvanic Series:
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 AI ) 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) Lead Tin Solder Cast iron Low-carbon steel (mild steel) Manganese Uranium Aluminum Alloys Cadmium Aluminum Zinc Beryllium Magnesium Note, positions of SS and Al
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.electrochemical cell electrolytewet celllead-acid batteries
- oxidation reaction that happens at zinc = anode : Zn(s) → Zn 2+ (aq) + 2 e - Dry Cell - Zinc-carbon battery - reduction reaction at carbon rod = cathode
Dissimilar metals are physically joined in the presence of an electrolyte. The more anodic metal corrodes. Galvanic Corosion Bilge pump - Magnesium shell cast around a steel core.
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.
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
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. 5th Century sword Boiler tube 304 stainless steel / acid chloride solution
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.
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!!
C orrosion along grain boundaries, often where precipitate particles form. 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.
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
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.
C ombined chemical attack and mechanical wear (e.g., pipe elbows) Erosion-corrosion 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.
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).
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!