Text of 8 Forms of Corrosion: Uniform Pitting Crevice Corrosion or Concentration Cell Galvanic or Two-Metal...
8 Forms of Corrosion: Uniform Pitting Crevice Corrosion or Concentration Cell Galvanic or Two-Metal Stress Corrosion Cracking Intergranular Dealloying Erosion Corrosion http://www.intercorr.com/failures.html
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 (Figure 13-1) 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.
Questions: 1.Worst combination? 2.Aluminum and steel? 3.Titanium and Zinc? 4.Stainless Steel and Copper? 5.Mild steel and cast iron? Galvanic Series: Show Demo!
GALVANIC SERIES Galvanic Series in Seawater (supplements Faraq Table 3.1, page 65), EIT Review Manual, page 38-2 Tendency to be protected from corrosion, cathodic, more noble end 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 2 0 3 ) 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 Note, positions of ss and al**
Big Cathode, Small Anode = Big Trouble
Figure 1 illustrates the idea of an electro-chemical reaction. If a metal is placed in a conducting solution like salt water, it dissociates into ions, releasing electrons, as the iron is shown doing in the figure, via the ionization reaction Fe Fe ++ + 2e - The electrons accumulate on the iron giving it a negative charge that grows until the electrostatic attraction starts to pull the Fe ++ ions back onto the metal surface, stifling further dissociation. At this point the iron has a potential (relative to a standard, the hydrogen standard) of 0.44 volts. Each metal has its own characteristic corrosion potential (called the standard reduction potential), as plotted in Figure 2. If two metals are connected together in a cell, like the iron and copper samples in Figure 1, a potential difference equal to their separation on Figure 2 appears between them. The corrosion potential of iron, -0.44, differs from that of copper, +0.34, by 0.78 volts, so if no current flows in the connection the voltmeter will register this Figure 1. A bi-metal corrosion cell. The corrosion potential is the potential to which the metal falls relative to a hydrogen standard. Figure 2. Standard reduction potentials of metals. Galvanic Corrosion Potentials:
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 carselectrochemical cellelectrolytewet celllead-acid batteries
Zn(s) Zn 2+ (aq) + 2 e - - oxidation reaction that happens at zinc = anode Dry Cell - Zinc-carbon battery 2MnO 2 (s) + 2 H + (aq) + 2 e - Mn 2 O 3 (s) + H 2 O(l) - reduction reaction at carbon rod = cathode
Design for Galvanic Corrosion? Material Selection: Do not connect dissimilar metals! Or if you cant 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. See handout! Read Payer video HO
Stress Corrosion Cracking: Spontaneous corrosion induced cracking of a material under static (or residual) tensile stress. Problem w/ parts that have residual stress stamping double whammy residual stress at bends = SCC + stress concentration. AKA environmentally assisted cracking (EAC), other forms: Hydrogen embrittlement Caustic embrittlement Liquid metal corrosion
Factors: Must consider metal and environment. What to watch for: Stainless steels at elevated temperature in chloride solutions. Steels in caustic solutions Aluminum in chloride solutions 3 Requirements for SCC: 1.Susceptible alloy 2.Corrosive environment 3.High tensile stress or residual stress
Design for Stress Corrosion Cracking: Material selection for a given environment (Table 13-2). Reduce applied or residual stress - Stress relieve to eliminate residual stress (i.e. stress relieve after heat treat). Introduce residual compressive stress in the service. Use corrosion alloy inhibitors. Apply protective coatings.
Stress Corrosion Cracking: See handout, review HO hydron!
Intergranular Attack: Corrosion which occurs preferentially at grain boundries. Why at grain boundries? Higher energy areas which may be more anodic than the grains. The alloy chemistry might make the grain boundries dissimilar to the grains. The grain can act as the cathode and material surrounding it the anode.
Intergranular Attack: How to recognize it? Near surface Corrosion only at grain boundries (note if only a few gb are attacked probably pitting) Corrosion normally at uniform depth for all grains.
Example 1: Intergranular Attack: Sensitization of stainless steels: Heating up of austenitic stainless steel (750 to 1600 F) 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.
Example: Intergranular Attack: Sensitization of stainless steels: Heating up of austenitic stainless steel (750 to 1600 F) 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!!
Design for Intergranular Attack: Watch welding of stainless steels (causes sensitization). Always anneal at 1900 2000 F 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.
Example 2: Intergranular Attack: Exfoliation of high strength Aluminum alloys. Corrosion that preferentially attacts the elongated grains of rolled aluminum. Corroded grains usually near surface Grain swells due to increase in volume which causes drastic separation to occur in a pealing fashion.
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: Danger! The alloy may not appear damaged May be no dimensional variations Material generally becomes weak hidden to inspection!
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).
Erosion: Basically a repeat of Chapter 3 (see- Erosion Wear) Forms of Erosion: Liquid Impingement Liquid erosion Slurry Erosion Cavitation
Methods to Control Corrosion There are five methods to control corrosion: material selection coatings changing the environment changing the potential design
How to avoid (or control) Corrosion? Material Selection! Remember environment key. Look at potential pH diagrams!!! Eliminate any one of the 4 reqments for corrosion! Galvanic - Avoid using dissimilar metals. Or close together as possible Or electrically isolate one from the other Or MAKE ANODE BIG!!!
How to avoid (or control) Corrosion? Pitting/Crevice: Watch for stagnate water/ electrolyte. Use gaskets Use good welding practices Intergranular watc