4.0 CORROSION PREVENTION MATERIAL SELECTION ALTERATION OF ENVIRONMENT PROPER DESIGN CATHODIC PROTECTION ANODIC PROTECTION COATINGS & WRAPPING
(1) MATERIAL SELECTION (selection of proper material for a particular corrosive service) • Metallic [metal and alloy] • Nonmetallic [rubbers (natural and synthetic), plastics, ceramics, carbon and graphite, and wood]
E.g : Stainless Steels Stainless steels are iron base alloys that contain a minimum of approximately 11% Cr, the amount needed to prevent the formation of rust in unpolluted atmosphere. Dissolution rate, cm/sec wt.% Cr
Alloying elements of stainless steel : • Other than Ni, Cr and C, the following alloying elements may also present in stainless steel: Mo, N, Si, Mn, Cu, Ti, Nb, Ta and/or W. • Main alloying elements (Cr, Ni and C): 1. Chromium Minimum concentration of Cr in a stainless steel is 12-14wt.% Structure : BCC (ferrite forming element) * Note that the affinity of Cr to form Cr-carbides is very high. Chromium carbide formation along grain boundaries may induce intergranular corrosion.
Binary diagram of Fe-Cr Sigma phase formation which is initially formed at grain boundaries has to be avoided because it will increase hardness, decrease ductility and notch toughness as well as reduce corrosion resistance.
2. Nickel Structure: FCC (austenite forming element/stabilize austenitic structure) Added to produce austenitic or duplex stainless steels. These materials possess excellent ductility, formability and toughness as well as weld-ability. Nickel improves mechanical properties of stainless steels servicing at high temperatures. Nickel increases aqueous corrosion resistance of materials.
Ternary diagram of Fe-Cr-Ni at 6500 and 10000C AISI : American Iron and Steel Institute
3. Carbon Very strong austenite forming element (30x more effective than Ni). I.e. if austenitic stainless steel 18Cr-8Ni contains ≤0.007%C, its structure will convert to ferritic structure. However the concentration of carbon is usually limited to ≤ 0.08%C (normal stainless steels) and ≤0.03%C (low carbon stainless steels to avoid sensitization during welding).
Minor alloying elements : • Manganese Austenitic forming element. When necessary can be used to substitute Ni. Concentration of Mn in stainless steel is usually 2-3%. • Molybdenum Ferritic forming element. Added to increase pitting corrosion resistance of stainless steel (2-4%). Molybdenum addition has to be followed by decreasing chromium concentration (i.e. in 18-8SS has to be decreased down to 16-18%) and increasing nickel concentration (i.e. has to be increased up to 10-14%). Improves mechanical properties of stainless steel at high temperature. Increase aqueous corrosion resistance of material exposed in reducing acid.
Tungsten Is added to increase the strength and toughness of martensitic stainless steel. • Nitrogen (up to 0.25%) Stabilize austenitic structure. Increases strength and corrosion resistance. Increases weld ability of duplex SS. • Titanium, Niobium and Tantalum To stabilize stainless steel by reducing susceptibility of the material to intergranular corrosion. Ti addition > 5x%C. Ta+Nb addition > 10x%C.
Copper Is added to increase corrosion resistance of stainless steel exposed in environment containing sulfuric acid. • Silicon Reduce susceptibility of SS to pitting and crevice corrosion as well as SCC.
Influence of alloying elements on pitting corrosion resistance of stainless steels
Influence of alloying elements on crevice corrosion resistance of stainless steels
Influence of alloying elements on SCC resistance of stainless steels
Five basic types of stainless steels : • Austenitic - Susceptible to SCC. Can be hardened by only by cold working. Good toughness and formability, easily to be welded and high corrosion resistance. Nonmagnetic except after excess cold working due to martensitic formation. • Martensitic - Application: when high mechanical strength and wear resistance combined with some degree of corrosion resistance are required. Typical application include steam turbine blades, valves body and seats, bolts and screws, springs, knives, surgical instruments, and chemical engineering equipment. • Ferritic - Higher resistance to SCC than austenitic SS. Tend to be notch sensitive and are susceptible to embrittlement during welding. Not recommended for service above 3000C because they will loss their room temperature ductility.
Duplex (austenitic + ferritic) – has enhanced resistance to SCC with corrosion resistance performance similar to AISI 316 SS. Has higher tensile strengths than the austenitic type, are slightly less easy to form and have weld ability similar to the austenitic stainless steel. Can be considered as combining many of the best features of both the austenitic and ferritic types. Suffer a loss impact strength if held for extended periods at high temperatures above 3000C. • Precipitation hardening - Have the highest strength but require proper heat-treatment to develop the correct combination of strength and corrosion resistance. To be used for specialized application where high strength together with good corrosion resistance is required.
Stress Corrosion Cracking of Stainless Steel • Stress corrosion cracking (SCC) is defined as crack nucleation and propagation in stainless steel caused by synergistic action of tensile stress, either constant or slightly changing with time, together with crack tip chemical reactions or other environment-induced crack tip effect. • SCC failure is a brittle failure at relatively low constant tensile stress of an alloy exposed in a specific corrosive environment. • However the final fracture because of overload of remaining load-bearing section is no longer SCC.
Three conditions must be present simultaneously to produce SCC: - a critical environment - a susceptible alloy - some component of tensile stress
Pure metals are more resistance to SCC but not immune and susceptibility increases with strength Tensile stress is below yield point Susceptible material Tensile stress Stress corrosion cracking Corrosive environment Corrosive environment is often specific to the alloy system
Typical micro cracks formed during SCC of sensitized AISI 304 SS Surface morphology
Example of crack propagation during transgranular stress corrosion cracking (TGSCC) brass
Example of crack propagation during intergranular stress corrosion cracking (IGSCC) ASTM A245 carbon steel
Fracture surface of intergranular SCC on carbon steel in hot nitric solution Fracture surface of transgranular SCC on austenitic stainless steel in hot chloride solution
Fracture surface due to local stress has reached its tensile strength value on the remaining section Fracture surface due to intergranular SCC
Electrochemical effect Usual region for TGSCC, mostly is initiated by pitting corrosion(transgranular cracking propagation needs higher energy) pitting Zone 1 cracking zones passive Zone 2 Usual region for IGSCC, SCC usually occurs where the passive film is relatively weak active
Note that non-susceptible alloy-environment combinations, will not crack the alloy even if held in one of the potential zones. • Temperature and solution composition (including pH, dissolved oxidizers, aggressive ions and inhibitors or passivators) can modify the anodic polarization behavior to permit SCC. • Susceptibility to SCC cannot be predicted solely from the anodic polarization curve.
Models of stress corrosion cracking • Slip step dissolution model • Discontinuous intergranular crack growth • Crack nucleation by rows of corrosion micro-tunnels • Absorption induced cleavage • Surface mobility (atoms migrate out of the crack tips) • Hydrogen embrittlement→HIC
Control/prevention : • Reduce applied stress level • Remove residual tensile stress (internal stress) • Lowering oxidizing agent and/or critical species from the environment • Add inhibitor • Use more resistant alloys • Cathodic protection
2. Alteration of Environment • Typical changes in medium are : • Lowering temperature – but there are cases where increasing T decreases attack. E.g hot, fresh or salt water is raised to boiling T and result in decreasing O2 solubility with T. • Decreasing velocity – exception ; metals & alloys that passivate (e.g stainless steel) generally have better resistance to flowing mediums than stagnant. Avoid very high velocity because of erosion-corrosion effects.
Removing oxygen or oxidizers – e.g boiler feedwater was deaerated by passing it thru a large mass of scrap steel. Modern practice – vacuum treatment, inert gas sparging, or thru the use of oxygen scavengers. However, not recommended for active-passive metals or alloys. These materials require oxidizers to form protective oxide films. • Changing concentration – higher concentration of acid has higher amount of active species (H ions). However, for materials that exhibit passivity, effect is normally negligible.
Environment factors affecting corrosion design : • Dust particles and man-made pollution – CO, NO, methane, etc. • Temperature – high T & high humidity accelerates corrosion. • Rainfall – excess washes corrosive materials and debris but scarce may leave water droplets. • Proximity to sea • Air pollution – NaCl, SO2, sulfurous acid, etc. • Humidity – cause condensation.
3. Design Do’s & Don’ts • Wall thickness – allowance to accommodate for corrosion effect. • Avoid excessive mechanical stresses and stress concentrations in components exposed to corrosive mediums. Esp when using materials susceptible to SCC. • Avoid galvanic contact / electrical contact between dissimilar metals to prevent galvanic corrosion. • Avoid sharp bends in piping systems when high velocities and/or solid in suspension are involved – erosion corrosion. • Avoid crevices – e.g weld rather than rivet tanks and other containers, proper trimming of gasket, etc.
Avoid sharp corners – paint tends to be thinner at sharp corners and often starts to fail. • Provide for easy drainage (esp tanks) – avoid remaining liquids collect at bottom. E.g steel is resistant against concentrated sulfuric acid. But if remaining liquid is exposed to air, acid tend to absorb moisture, resulting in dilution and rapid attack occurs. • Avoid hot spots during heat transfer operations – localized heating and high corrosion rates. Hot spots also tend to produce stresses – SCC failures. • Design to exclude air – except for active-passive metals and alloys coz they require O2 for protective films. • Most general rule : AVOID HETEROGENEITY!!!
4. Protective Coatings / Wrapping • Provide barrier between metal and environment. • Coatings may act as sacrificial anode or release substance that inhibit corrosive attack on substrate. • Metal coatings : • Noble – silver, copper, nickel, Cr, Sn, Pb on steel. Should be free of pores/discontinuity coz creates small anode-large cathode leading to rapid attack at the damaged areas. • Sacrificial – Zn, Al, Cd on steel. Exposed substrate will be cathodic & will be protected. • Application – hot dipping, flame spraying, cladding, electroplating, vapor deposition, etc.
Surface modification – to structure or composition by use of directed energy or particle beams. E.g ion implantation and laser processing. • Inorganic coating : cement coatings, glass coatings, ceramic coatings, chemical conversion coatings. • Chemical conversion – anodizing, phosphatizing, oxide coating, chromate. • Organic coating : paints, lacquers, varnishes. Coating liquid generally consists of solvent, resin and pigment. The resin provides chemical and corrosion resistance, and pigments may also have corrosion inhibition functions.
5. Cathodic and Anodic Protection5.1: Cathodic Protection • Cathodic Protection (CP) was employed before the science of electrochemistry had been developed; • CP is achieved by supplying electrons to the metal structure to be protected. (M Mn+ + ne) and (2H+ + 2e H2); • Examination of equation indicates the addition of electrons to the structure will tend to suppress metal dissolution and increase the rate of hydrogen dissolution; • If current is considered to flow from (+) to (-), then a structured is protected if current enters it from the electrolyte; • Conversely, accelerated corrosion occurs if current passes from the metal to the electrolyte, this current convention has been adopted in cathodic protection technology and is used here for consistency;