CHAPTER 3 FORMS OF CORROSION Chapter Outlines 3

CHAPTER 3 FORMS OF CORROSION Chapter Outlines 3. 1 3. 2 3. 3 3. 4 Galvanic or Two-Metal Corrosion Crevice Corrosion Pitting Corrosion Intergranular Corrosion 1

GALVANIC CORROSION 2

3. 1 Galvanic or Two-Metal Corrosion q also called ' dissimilar metal corrosion‘. q Takes place when two metals are in physical contact with each other and are immersed in a conducting fluid. q corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. q Examples: 1. Plate and screw of different electrical potentials due to differences in processing 2. Multiple component implant using different metals for each component 3. Copper and steel tubing are joined in a domestic water heater, the steel will corrode in the vicinity of the junction q The following fundamental requirements have to be met for galvanic corrosion: 1. Dissimilar metals (or other conductors, such a graphite). 2. Electrical contact between the dissimilar conducting materials (can be direct contact or a secondary connection such as a common grounding path). 3 3. Electrolyte (the corrosive medium) in contact with the dissimilar conducting materials.

The relative nobility of a material can be predicted by measuring its corrosion potential. The well known galvanic series lists the relative nobility of certain materials in sea water. A small anode/cathode area ratio is highly undesirable. In this case, the galvanic current is concentrated onto a small anodic area. Rapid thickness loss of the dissolving anode tends to occur under these conditions. Galvanic corrosion problems should be solved by designing to avoid these problems in the first place. Fig. Galvanic corrosion between stainless steel screw and Aluminium. Fig. Galvanic corrosion between Steel and Brass. Noble, cathodic end Platinum Gold Graphite Titanium Silver Hastelloy C 18 -8 austenitic stainless steels (passive condition) Iron-chromium alloys (passive condition) Inconel (passive) Nickel Monel Cupronickel alloys Bronzes Copper Brasses Inconel (active) Nickel (active) Tin Lead 18 -8 Austenitic stainless steels (active) 13% Chromium stainless steel (active) Cast iron Mild steel and iron Cadmium Aluminum alloys Zinc Magnesium and magnesium alloys Active, anodic end Fig. Anodic- cathodic behavior of steel with zinc and tin outside layers exposed to the Atmosphere. (a) zinc is anodic to steel and corrodes (b) steel is anodic to tin and corrodes (the tin 4 layer was perforated before the corrosion began)

The following have been described as "main factors" factors influencing galvanic corrosion rates in Skanaluminium's online publication "Alubook - Lexical knowledge about aluminium". • Potential Difference between materials • Cathode Efficiency • Surface areas of connected materials (area ratio) • Electrical resistance of the connection between the materials and of the electrolyte. Note that the area ratio of the anode: cathode is an important variable affecting the dissolution current density (and hence corrosion rate) pertaining to the anode. The area ratio is also important when considering the relative amount of current "available" from the cathodic reaction. Fig. Brass on Weathering Steel - rust forms in discrete Steel crystallites that are fine, red and diffusely reflecting, like hematite. The massive re-crystallized layer is a shiny blue, 5 approaching the blue-black of secular hematite.

CREVICE CORROSION 6

Rivets : a metal pin for passing through holes in two or more plates or pieces to hold them together, usually made with a head at one end, the other end being hammered into a head after insertion. Depletion : to decrease seriously or exhaust the abundance or supply of 3. 2 Crevice Corrosion q Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. q Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps. q Occurs under gaskets, rivets and bolts, between valve disks and seats. q Well-known examples of such geometries including flanges, gaskets, disbonded linings/coatings, fasteners, lap joints and surface deposits. Crevice corrosion is initiated by changes in local chemistry within the crevice: • • Depletion of inhibitor in the crevice Depletion of oxygen in the crevice A shift to acid conditions in the crevice Build-up of aggressive ion species (e. g. chloride) in the crevice 7

Mechanism- Chronology of Crevice Corrosion Stage 1 At time zero, the oxygen content in the water occupying a crevice is equal to the level of soluble oxygen and is the same everywhere. Stage 2 Because of the difficult access caused by the crevice geometry, oxygen consumed by normal uniform corrosion is very soon depleted in the crevice. The corrosion reactions now specialize in the crevice (anodic) and on the open surface (cathodic). Fig. Schematic illustration (initial stage) of the mechanism for crevice corrosion between two riveted sheets. Stage 3 The crevice development a few more accelerating factors fully develop: 1. The metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons (acid) and forming corrosion products. The p. H in a crevice can reach very acidic values, sometimes equivalent to pure acids. 2. The acidification of the local environment can produce a serious increase in the corrosion rate of most metals. See, for example, how the corrosion of steel is affected as a function of water p. H. 3. The corrosion products seal even further the crevice environment. 8 4. The accumulation of positive charge in the crevice becomes a strong attractor to negative ions in the environment, such as chlorides and sulfates, that can be corrosive in their own right.

Fig. Crevice corrosion on aircraft Fig. Pack rust is a form a localized corrosion typical of steel components that develop a crevice into an open atmospheric environment. This expression is often used in relation to bridge inspection to describe built-up members of steel bridges which are showing signs of rust packing between steel plates. Zebra mussels- an example of marine environment 9 Close-up picture showing the severity of corrosion Underside of panel where severe corrosion was found

Crevice Corrosion Testing ASTM G 78 Standard Guide for Crevice Corrosion Testing A good example of how crevice corrosion can be reproduced and accelerated in a laboratory environment is the formation of occluded cells with multiple crevice assemblies (MCAs), as described in the ASTM G 78 Standard Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments. In this test, washers make a number of contact sites on either side of the specimens. The number of sites showing attack in a given time can be related to the resistance of a material to initiation of localized corrosion, and the average or maximum depth of attack can be related to the rate of propagation. The large number of sites in duplicate or triplicate specimens is amenable to probabilistic evaluation. The susceptibility to localized corrosion becomes quite visible once a specimen equipped with these Teflon washers has been exposed to a corrosive environment for an extended period of time. . after 30 days in 0. 5 Fe. Cl 3 + 0. 05 M 10 Na. Cl

PITTING CORROSION 11

3. 3 Pitting Corrosion q Pitting corrosion is a localized form of corrosive attack that produces holes or small pits in a metal. q the bulk of the surface remains unattacked. q Pitting is often found in situations where resistance against general corrosion is conferred by passive surface films. q Localized pitting attack is found where these passive films have broken down. q Pitting attack induced by microbial activity, such as sulfate reducing bacteria (SRB) also deserves special mention. q Pitting can be one of the most dangerous forms of corrosion because it is difficult to anticipate and prevent, relatively difficult to detect, occurs very rapidly, and penetrates a metal without causing it to lose a significant amount of weight. Special case of crevice corrosion: 1. 2. Initiated by inclusions, scratches, or handling damage instead of deep cracks The presence of static flow conditions and reduced oxygen availability are less important than in crevice corrosion 12

Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products. Pits can be either hemispherical or cup-shaped. Pitting is initiated by: by 1. 2. 3. Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater) Localized damage to, or poor application of, a protective coating. The presence of non-uniformities in the metal structure of the component, e. g. nonmetallic inclusions. A local cell that leads to the initiation of a pit can be caused by an abnormal anodic site surrounded by normal surface which acts as a cathode, or by the presence of an abnormal cathodic site surrounded by a normal surface in which a pit will have disappeared due to corrosion. Fig. Local Cathode on a corroded piece of material 13

q Alloying can have a significant impact on the pitting resistance of stainless steels. q Conventional steel has a greater resistance to pitting than stainless steels, but is still susceptible, especially when unprotected. q Aluminum in an environment containing chlorides and aluminum brass (Cu-20 Zn-2 Al) in contaminated or polluted water are usually susceptible to pitting. q Titanium is strongly resistant to pitting corrosion. q Proper material selection is very effective in preventing the occurrence of pitting corrosion. Another option for protecting against pitting is to mitigate aggressive environments and environmental components (e. g. chloride ions, low p. H, etc. ). q Inhibitors may sometimes stop pitting corrosion completely. q Further efforts during design of the system can aid in preventing pitting corrosion, for example, by eliminating stagnant solutions or by the inclusion of cathodic protection. FIG The pitting of a 304 stainless steel plate by an acid-chloride solution. 14

Sewer Explosion due to Pitting Corrosion Fig. Corrosion Pits are the primary source of leaks in water handling systems Pitting: corrosion of a metal surface, confined to a point or small area, that takes the form of cavities. Pitting factor: ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss. Pitting resistance equivalent number (PREN): an empirical relationship to predict the pitting resistance of austenitic and duplex stainless steels. It is expressed as PREN = Cr + 3. 3 (Mo + 0. 5 W) + 16 N An example of corrosion damages with shared responsibilities was the sewer explosion that killed 215 people in Guadalajara, Mexico, in April 1992. Besides the fatalities, the series of blasts damaged 1, 600 buildings and injured 1, 500 people. *Sewer: an artificial conduit, usually underground, for carrying off waste water and refuse, as in a town or city. This example of a pitted surface was produced by exposing a specimen of aluminum A 92519 to 3. 5% Na. Cl during seven days. The width of the picture is approximately 1 mm. 15

Corrosion Pit Shapes SIDEWAY PITS THROUGH PITS Subsurface Narrow, deep Shallow, wide Undercutting Elliptical Horizontal grain attack Vertical Grain Attack 16

Table 1 Pitting corrosion incidents of aircraft and helicopters Aircraft Location of Failure Cause Bell Helicopter Fuselage, longeron Fatigue, corrosion and pitting present DC-6 Engine, master connecting rod Piper PA-23 Incident Severity Place Year From Serious AR, USA 1997 NTSB Corrosion pitting Fatal AK, USA 1996 NTSB Engine, cylinder Corrosion pitting Fatal AL, USA 1996 NTSB Boeing 75 Rudder Control Corrosion pitting Substantial damage to plane WI, USA 1996 NTSB Embraer 120 Propeller Blade Corrosion pitting Fatal and serious, loss of plane GA, USA 1995 NTSB Gulfstream GA-681 Hydraulic Line Corrosion pitting Loss of plane, no injuries AZ, USA 1994 NTSB L-1011 Engine, compressor assembly disk Corrosion pitting Loss of plane, no injuries AK, USA 1994 NTSB Embraer 120 Propeller Blade Corrosion pitting Damage to plane, no injuries Canada 1994 NTSB Embraer 120 Propeller Blade Corrosion pitting Damage to plane, no injuries Brazil 1994 NTSB F/A-18 Trailing-edge Flap (TEF) Outboard Hinge Lug Corrosion pitting, fatigue Loss of TEF Australia 1993 AMRL 17

INTERGRANULAR CORROSION 18

3. 4 Intergranular Corrosion q Intergranular corrosion refers to preferential (localized) corrosion along grain boundaries. q or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. q This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated on the grain boundaries. q This selective dissolution may lead to the dislodgement of grains. q Intergranular corrosion in sensitized stainless steels and exfoliation in aluminum alloys represent industrially significant examples of this form of damage. ü ü Also known as “knife- line attack” classic example is the sensitization of 19 stainless steels or weld decay

Fig. Severe problem in the welding of stainless steels, when it is termed weld decay. FIG. Intergranular corrosion of a failed aircraft component made of 7075 -T 6 aluminum ( picture width = 500 mm) 20