Materials & Corrosion

1. Materials & Corrosion

2. Why worry about corrosion? “One large chemical company spent more than $400,000 per year for corrosion maintenance in its sulfuric acid plants, even though the corrosion conditions were not considered to be particularly severe.”

3. Why worry about corrosion? “A refinery employing a new process developed a serious problem after just 16 weeks of operation; some parts showed a corrosion loss of as much as 1/8 inch.”

4. Why worry about corrosion? “The trend in the chemical process industries toward higher temperatures and pressures has made possible new processes or improvements in old processes. Higher temperatures and pressures usualy involve more severe corrosion conditions. Many of the present day operations would not have been possible or economical w/o the use of corrosion-resistant materials.”

5. Why Worry about Corrosion? • Safety/Health • Environment • Operability • Profitability • Product Quality • Appearance of Facilities/Equipment • Badly corroded and rusted equipment in a plant would leave a poor impression on the observer

6. Examples of Corrosion • Erosion-corrosion of copper water pipe (tubing) • Chloride Stress corrosion cracking of stainless steels • Chloride pitting of stainless steel • Nitric acid attack of titanium tubing

7. Corrosive Environments • Atmospheric • Industrial • Urban • Rural • Marine • Water • Seawater • Freshwater • Tapwater • Treated water

8. Corrosive Environments • Soil • Industrial – commercial/institutional

9. Definition of corrosion • Is the chemical or electro-chemical reaction between a material – usually a metal – ant it’s environment that produces a deterioration of the material and it’s properties.

10. Factors Influencing Corrosion • Design, fabrication & materials • Design/configuration • Forming, welding, heat treating • Metallurgy, composition • Environmental factors • Process chemistry including: • pH • Dissolved materials • Salt • Metal ions • Gases (O2, N2, CO2, ammonia, chlorine etc.) • Suspended matter • Temperature • Flow velocity • Micro-organisms

11. Types of Corrosion 2 Main Types: • General corrosion is the loss material over the entire surface, at a relatively constant rate. The metal is thinned fairly uniformly, without appreciable localized attack. • Localized Attack is specific to certain areas of the material, and can take many forms.

12. Forms of Localized Attack • Pitting is corrosion that produces sites of localized attack that are small relative to the overall exposed surface area. It is the most common form of corrosion in aqueous environments, and the major cause of corrosion service failures in the chemical processing industry.

13. Forms of Localized Attack cont’d • CREVICE CORROSION is attack at narrow spaces or gaps between two metal surfaces, or between a metal and non-metal. It can occur at cracks or seams in a metal, or under a washer, gasket, or deposit. Crevice corrosion results from a difference between the chemistry of the bulk environment and that in or at the crevice.

14. Forms of Localized Attack cont’d • BIOLOGICAL ATTACK is that which caused or accelerated by organisms on the affected surface. Fouling organic deposits may cause crevice corrosion, or organisms may produce chemicals that cause corrosion.

15. EROSION CORROSION is acceleration of material loss due to the combined effects of corrosion plus removal of material by the moving fluid.

16. Forms of Localized Attack cont’d • CAVITATION & IMPINGEMENT is material loss due to collapse of voids or cavities in the fluid, due to pressure changes (cavitation), or due to impingement of liquid droplets. This may be strictly mechanical damage, or may be worsened by the effects of corrosion.

17. S.E.M. What is it?

18. Forms of Localized Attack cont’d • Fretting is a process combining wear and corrosion in removal of material from contacting solid surfaces. It typically involves very small relative movements of the components, oxidation of the surfaces, and abrasion by the oxidation products. It often occurs between a shaft and a component fitted on the shaft.

19. Forms of Localized Attack cont’d • INTERGRANULAR ATTACK is corrosion at the boundaries of a metal grain, with little or no attack of the grain. It results in weakening of the metal, or separation at the grain boundary. (The composition and corrosion resistance of a metal grain varies from the surface to the interior.)

20. Forms of localized Attack cont’d • DEALLOYING ATTACK is preferential removal of one constituent of an alloy in a corrosive environment. An example is the “dezincification” of brass, in which zinc is leached from the brass in some aqueous streams, leaving a weak structure of copper and copper oxide.

21. Forms of Localized Attack cont’d • GALVANIC ATTACK is attack of a metal that is in electrical contact with a more noble metal, or a non-metallic conductor, in a corrosive environment. Examples are: corrosion of copper or brass couple to steel in an aqueous environment: or corrosion of a zinc coating on steel. The latter is done intentionally to protect the steel, as in roofing nails, fencing, corrugated galvanized sheets, etc.

22. Types of Corrosion Environmentally-induced cracking: • Stress-corrosion cracking (SCC) is due to the combined effects of a corrodent and sustained tensile stress. SCC of AUSTENTITIC (300-series) stainless steels by chorides is a major problem in the chemical industry. SCC can be caused in copper alloys in nitrates, and in steel by caustic.

23. Environmentally-induced Cracking • CORROSION FATIGUE occurs in a cyclically loaded part in a corrosive environment. It occurs at lower stress levels or after fewer cycles that would be the case in the absence of the corrosive environment.

24. Environmentally-induced Cracking • Hydrogen-induced cracking or Embrittlement is reduction of the ductility or toughness of a metal due to the presence of atomic hydrogen. The hydrogen can be present due to introduction into the molten metal, or through absorption by the solid metal.

25. Environmentally-induced Cracking cont’d • LIQUID METAL EMBRITTLEMENT is brittle failure of a normally ductile metal when coated with a thin film of liquid metal followed by stressing in tension. Examples of LME may occur when steel is brazed, soldered, welded, or plated, or dip-coated with zinc, cadmium, or tin.

26. Corrosion Control • Materials selection • Design • Method of Operation • Barriers/coatings • Paint systems • Linings – e.g. rubber or plastic • Cladding – metal on metal • Galvanizing • Plating • Glass/ceramic – e.g. porcelain on steel • Inhibitors • Cathodic/Anodic Protection

27. Factors affecting choice of an engineering material Strength Appearance Corrosion resistance Materials Selection Availability Fabricability Cost

28. Corrosion-resistant Materials All materials are resistant to corrosion in some specific environments. For example, carbon steel is resistant to many process and aqueous liquids. It may corrode slowly or not at all. Steel is the main material used in chemical plant equipment. The term corrosion-resistant is used to refer to materials that resist attack in specific or unusually corrosive environments.

29. Corrosion-Resistant Materials The following is a list of a few materials used in various corrosive services: • Stainless steels • Nickel & nickel alloys • Reactive & refractory metals such as tantalum, titanium, zirconium, & their alloys • Copper/copper alloys (brasses, bronzes)

30. Corrosive-Resistant Materials • Aluminum • Lead • Chromium • Plastics, such as teflon, PVC, nylon, polypropylene, polyethylene, etc. • Rubbers/elastomers such as nitrile (NBR or BUNA-N) EPDM (e.g. NORDEL), Viton, NEOPRENE, butyl, etc. • COMPOSITES, e.g. fiber-reinforced plastic (FRP, fiberglass) • GLASS & CERAMICS, tile, porcelain, brick

31. STAINLESS STEELS • Iron-based alloys, with at least 10.5% Chromium • Chromium-rich oxide surface film • “passive” • Forms and heals in oxygen • Additives of nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulphur, selenium • Processing applications, cutlery, decorative, health and sanitary, dairy, transportation, medical

32. STAINLESS STEELS FERRITIC • Iron plus 10-25% chromium (BCC) • Magnetic • High strength, limited toughness • Good ductility & fabricability • Examples : 405,409, 429, 430, 434, 442, monit

33. STAINLESS STEELS AUSTENTITIC • Iron plus chromium plus nickel (FCC) • Also molybdenum, manganese, copper, nitrogen, others • Non-magnetic (Usually) • Relatively low strength • High ductility/toughness • Susceptible to chloride attack (SCC, pitting) • Good fabricability • Examples: 201, 202, 205, 301, 302, 303, 304, 304L, 305, 308, 309, 310,316, 316L, 317, 321, 330, 347, 348, 384, 904L, nitronic types, Alloy 20, 20Cb-3, sanicro 28

34. STAINLESS STEELS DUPLEX • Mixed structure of BCC ferrite & FCC austenite • High corrosion resistance, SCC-resistant • High strength, good toughness • Examples: 329, 2205, 2507, 3RE60, 7-Mo PLUS, Ferralium 255

35. STAINLESS STEELS MARTENSITIC • Martensitic structure (distorted BCC) • Lower chromium: 10-18 % • Higher carbon: 0.15% min • Magnetic, hardenable • High strength – limited toughness • Lower corrosion resistance • Examples: 403, 410, 414, 416, 420, 431, 440

36. STAINLESS STEELS PRECIPITATION HARDENING • Chromium-nickel grades with copper, aluminum, titanium additions • Hardened by ageing at moderately elevated temperatures • Good formability (form then harden) • Susceptible to heat damage • Good corrosion resistance • Examples: 15-5 PH, 17-4 PH, 17-7 PH, PH 14-4 Mo, Custom 450, custom 455

37. OTHER ALLOYS COPPER & IT’S ALLOYS Copper • 99+% copper • Water service, marine, heat exchangers, architectural • Not heat treatable • Resists biofouling

38. COPPER & IT’S ALLOYS BRASSES • 5-45% Zinc • Dealloying above 15% zinc • Tin added to stop dealloying (admiralty and naval brasses)

39. COPPER & IT’S ALLOYS BRONZES • Phosphour bronze – 10% tin • Silicon bronze – 1-3% Si • Aluminum bronze – 5-10% Al

40. COPPER & IT’S ALLOYS CUPRO-NICKELS • 90/10 Cu/Ni, 80/20, 70/30 • Resistant to seawater, erosion, pitting

41. ALUMINUM & IT’S ALLOYS • Protected by barrier oxide film • Corrodes at low and high pH • Resistant to nitric acid (oxidizing) 1xxx – 99+% Al 2xxx • alloyed with copper • Strong, heat-treatable • Lower corrosion resistance

42. ALUMINUM & IT’S ALLOYS 4xxx – alloyed with Si 5xxx – Al-Mg-Mn, Al-Mg-Cr, Al-Mg-Mn-Cr

43. NICKEL & IT’S ALLOYS NICKEL • Alloy 200, commercially pure • Plating/cladding • Resistant to caustic MONEL • Alloy 400, approx. 30% copper • Very good fabricability

44. NICKEL & IT’S ALLOYS NICKEL-MOLY • Hastelloy B, B-2 • Resistant to HCI NICKEL-CHROMIUM • Inconel or Alloy 600 (77% Ni-15%Cr-bal Fe)

45. NICKEL & IT’S ALLOYS Ni-Fe-Cr • Incoloy or Alloy 800 (21Cr-32Ni-bal Fe) • Resistant to chlorides Ni-Fe-Cr-Mo • Includes Alloys 20 & 20Cb3, Incoloy 825, Hastelloy F & G • Increased resistance to sulphuric, phosphoric, and organic acids and to SCC and chloride pitting

46. NICKEL & IT’S ALLOYS Ni-Cr-Mo • Hastelloy C, C276, C-22 • Inconel or Alloy 625 • Resistant to hot acid mixtures

47. NON-METALLICS PLASTICS Thermoplastics • Polyethylene, PP, ABS, PVC, CPVC, PVDC • Fluorocarbons – PTFE or teflon, FEP, PFA, CTFE etc. • Nylon • Acetals, acrylics, polycarbonates, styrenes

48. Plastics Cont’d Thermosetting resins • Usually reinforced with glass • Epoxy, phenolics, polyester, furanes Rubber & Elastomers • Synthetic & natural rubbers Carbon & graphite Glass Ceramics & Refractories Concrete Wood

49. METALS

50. Non-metals From "Analysis, Synthesis, and Design of Chemical Processes" Turton, Bailie, Whiting, Shaeiwitz