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

Uniform Corrosion Most common – i.e. steel exposed to environment. Uniform in nature – leaves scale or deposit over entire exposed area – this is called rust which is really iron-oxide – Fe(OH)3 or Fe2O3 Fairly predictable and therefore the effects can be minimized! i.e. corrosion proportional to current, proportional to time (corrosion rate) < 2 mils/yr – necessary for food containment 20 mils/yr = conservative estimate for general atmospheric corrosion. Really general form of galvanic corrosion – i.e. anode and cathode random and in same material! Prevented by Removing electrolyte (i.e. lower relative humidity below 30%) Choose material that doesn’t rust in a particular environment – look at potential-pH diagram! Add design “allowance” for rust

Uniform (or general) corrosion of steel in water:

Uniform Corrosion Corrosion penetration rate (mils/yr): Constant depending on desired units Weight loss after exposure time t Exposure time density Exposed area

Uniform Corrosion: Corrosion rate in terms of current: r = rate in terms of mol/m2-s i = current per unit surface area of material corroding N = # of electrons associated with ionization of metal ion F = constant = 96,500 C/mol

A rate of less than 2 MPY is necessary for food containers A rate of less than 20 MPY for many industrial applications

EXAMPLE 1: MIG Welding tank METAL: Carbon Steel ENVIRONMENT: Industrial en FORM OF CORROSION: General METHOD TO CONTROL! Surface is painted for protection

EXAMPLE 3: Machine Shop Table METAL: Carbon Steel ENVIRONMENT: Industrial en FORM OF CORROSION: General METHOD TO CONTROL! Surface is painted for protection. Aggressive environment (molds dragged across surface) led to scrapping off of paint. Note corrosion where paint is scraped off in line.

EXAMPLE 4: Dumbbell METAL: Cast Iron ENVIRONMENT: Indoor (exercise room) FORM OF CORROSION: General METHOD TO CONTROL! Surface is painted for protection. Note, portion of dumbbell where paint was abraded off due to handling shows significant corrosion while areas that are better protected from abrasion retained paint and therefore show little corrosion.

EXAMPLE 5: House Drain and Drain Cap 1 year old cap 30 year old cap METAL: Cast Iron ENVIRONMENT: Residential basement – water exposure FORM OF CORROSION: General METHOD TO CONTROL! Surface is painted for protection. Note the 1 year old cap shows significant corrosion already!

60 YEAR OLD OIL PUMP

Kinzua Viaduct – see photos!! (1882/1900)

Crevice or Concentration Cell Local attack (corrosion) in crevice due to change in chemistry of electrolyte making it more aggressive – i.e. stagnant fluid = lower oxygen concentration = decrease in pH. Can be between metal surfaces or non-metal surfaces in contact with metal. Very destructive since highly localized! How design around? Leak proof weld Better gasket design Avoid stagnant water

Crevice or Concentration Cell Good example – crevices and recesses or under deposits of dirt or corrosion products where the solution is stagnet. Crevice must be wide enough to allow solution to penetrate yet narrow enough for stagnancy (i.e. few thousandths of an inch).

Crevice or Concentration Cell Depending on the environment developed in the crevice and the nature of the metal, the crevice corrosion can take a form of: pitting (i.e., formation of pits), filiform corrosion (this type of crevice corrosion that may occur on an aluminium surface underneath an organic coating), intergrannular attack, or stress corrosion cracking.

KEY – In crevice there are high concentrations of H+ and Cl- ions which are especially corrosive!

Crevice corrosion between pipe and I-beam: Rubber pads just accelerated the attack – why???

Track Fastener - Taipei EXAMPLE 2: Track Fastener - Taipei METAL: Ductile Cast Iron per ASTM D412 ENVIRONMENT: Corrosive Salt Water (Salt Spray) FORM OF CORROSION: Crevice Corrosion + General METHOD TO CONTROL! Surface is degreased, sand blasted and phosphatized for corrosion protection Surface is then painted with Chemlock elastomer primer and bonding adhesive. Note: Salt water trapped between elastomer and steel led to crevice corrosion which led to underbond corrosion. The adhesive to metal bond then failed causing the elastomer to delaminate. Resulted in return of several million dollar worth of product + replacement costs (labor and components).

Pitting Corrosion: Extremely localized corrosion that leads to the creation of small holes in the metal surfaces The driving power again is the lack of oxygen around a small area. This area becomes anodic while the area with excess of oxygen becomes cathodic. More of a problem in stagnant solutions. Very destructive since highly localized. Prevention? Material selection Avoid stagnant flow

Pitting Corrosion: Similar in chemistry to crevice corrosion except it happens in pits. Occurs in “pits” of metal surfaces where again, electrolyte is aggressive (stagnant). More of a problem in stagnant solutions. Very destructive since highly localized – may go undetected until failure occurs. Gravity causes pit to grow downward – corrosion rate can increase with time

Pitting Corrosion: A pit can be initiated by a localized surface defect, scratch or slight variation in composition. Stainless steels are especially susceptable to this form of corrosion. Prevention? Material selection Avoid stagnant flow Alloy SS with about 2% molybdenum.

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. *Meet requirements of a corrosion cell!! See slide 4

Galvanic Series:

Note, positions of ss and al** 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 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

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. See handout! – Read Payer video HO

Comparison of EMF series and galvanic series   EMF Series Galvanic Series absolute  relative quantitative qualitative pure metals only  metals & alloys half-cell potential  corrosion potential standard conditions  any specified conditions based on thermodynamic analysis  based on thermodynamic analysis used for theoretical calculations  used for practical applications

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. 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: Susceptible alloy Corrosive environment 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: 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. Mostly in the material chemistry.

Intergranular Attack:

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: Forms of Erosion: Liquid Impingement Liquid erosion Slurry Erosion Cavitation Read definitions on page 523.

EXAMPLES of UNIFORM CORROSION

Examples of Galvanic Corrosion

How to avoid (or control) Corrosion? Material Selection! Remember – environment key. Look at potential pH diagrams!!! Eliminate any one of the 4 req’ments for corrosion! Galvanic - Avoid using dissimilar metals. Or close together as possible Or electrically isolate one from the other Or MAKE ANODE BIG!!!

Methods to Control Corrosion There are five methods to control corrosion: material selection coatings changing the environment changing the potential design retyped

How to avoid (or control) Corrosion? Material Selection! Remember – environment key. Look at potential pH diagrams!!! Eliminate any one of the 4 req’ments 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 – watch grain size, environment, temperature, etc.. Careful with Stainless Steels and AL.

How to avoid (or control) Corrosion? Consider organic coating (paint, ceramic, chrome, etc.) – DANGER IF IT GETS SCRACTHED!! OR BETTER YET, consider cathodic protection: such as zinc (or galvanized) plating on steel Mg sacrificial anode on steel boat hull Impressed current, etc..

Corrosion Control: Anodic Protection – Zinc coating of steel. KNOW HOW THIS WORKS!!

DESIGN for Corrosion

DESIGN for Corrosion Bracket easier to replace than pipe!