CHAPTER 3 FORMS OF CORROSION (cont’)

1. CHAPTER 3 FORMS OF CORROSION (cont’) Chapter Outlines 3.5 Selective Leaching 3.6 Erosion Corrosion 3.7 Stress Corrosion 3.8 Hydrogen Damage

2. SELECTIVE LEACHING

3. SELECTIVELEACHING(“Dealloying”,“Parting”) Corrosion in which one constituent of an alloy is preferentially removed, leaving behind an altered (weakened) residual structure. Can occur in several systems.

4. Combinations of alloys and environments subject to dealloying and elements preferentially removed

5. Dezincification All Cu-Zn alloys (Brasses) containing> 15% Zn are susceptible . . . e.g. common yellow brass . . . 30 Zn 70 Cu, dezincifies to red copper-rich structure. Dezincification can be uniform... • potable water inside • or plug-type....(boiler water inside, combustion gases outside) Uniform dezincification of brass pipe. Plug-type dezincification.

6. Overall dimensions of original material tend to be retained . . . residual is spongy and porous . . . often brittle. Can go unnoticed, especially if covered with dirt/deposit, etc. Uniform dezincification... - usually found in high brasses (high[Zn]), acid environments; Plug-type dezincification... - usually found in low brasses, alkaline, neutral or slightly acid environments. Section of one of the plugs shown before

7. Prevention - Make environment less aggressive (e.g., reduce O2 content); - Cathodically protect; - Use a better alloy (common cure - above not usually feasible)... - “red” brass (<15% Zn) almost immune - Admiralty Brass. . . 70 Cu, 29 Zn, 1 Sn; - arsenical Admiralty. . .70 Cu, 29 Zn, 1 Sn, 0.04 As (Sn and Sn-As in deposited films hinder redeposition of Cu); - For very corrosive environments likely to provoke dezincification, or for critical components, use . . . - cupronickels 70-90 Cu, 30-10 Ni.

8. “Graphitization” (misnomer . . . graphitization is the breakdown of pearlite to ferrite + C at high temperature) Greycast iron is the cheapest engineering metal . . .2-4% C, 1-3% Si. Hard, brittle, easily cast; carbon present as microscopic flakes of matrix graphite within microstructure. Microstructure of grey cast iron. 100 m

9. In some environments (notably mild, aqueous soils affecting buried pipe) the Fe leaches out slowly and leaves graphite matrix behind . .appears graphitic . . .soft . . . can be cut with a knife. Pores usually filled with rust. Original dimensions are retained. A 200-mm (8-in.) diameter grey-iron pipe that failed because of graphitic corrosion. The pipe was part of a subterranean fire control system. The external surface of the pipe was covered with soil; the internal surface was covered with water. Severe graphitic corrosion occurred along the bottom external surface where the pipe rested on the soil. The small-diameter piece in the foreground is a grey-iron pump impeller on which the impeller vanes have disintegrated because of graphitic corrosion.

10. (a)External surface of a grey-ironpipe exhibiting severegraphitic corrosion. (b)Close-up of the graphitically-corrodedregion shown in (a). (c)Micrograph of symmetrical envelopes of graphitically-corroded iron surrounding flakes of graphite. 20 m

11. Selective Dissolution in Liquid Metals In liquid metal coolants (LMFBR with Na or Na-K coolant), austenitic alloys can lose Ni and Cr and revert to the ferrite phase... Corrosion of Inconel* alloy 706 exposed to liquid sodium for 8,000 hours at 700C (1290F); hot leg circulating system. A porous surface layer has formed with a composition of  95% Fe, 2% Cr and < 1% Ni. The majority of the weight loss encountered can be accounted for by this surface degradation. Total damage depth: 45 m. (a) Light micrograph. (b) SEM of the surface of the porous layer. * Alloy 706 ... 39-44% Ni, 14.5-17.5% Cr, 0.06% C.

12. Also in fusion-reactor environments (Li as coolant).... Light micrograph of cross-section. SEM of surface showing porous layer. Corrosion of type 316 stainless steel exposed to thermally convective lithium for 7488 hours at the maximum loop temperature of 600C.

13. Usually, the transport and deposition of leached elements is of more concern than the actual corrosion. (a)(b) SEM micrographs of chromium mass transfer deposits found at the 460C (860C) position in the cold leg of a lithium/type-316-stainless-steel thermal convection loop after 1700 hours. Mass transfer deposits are often a more serious result of corrosion than wall thinning. (a) Cross section of specimen on which chromium was deposited. (b) Top view of surface.

14. 100 m Iron crystals found in a pluggedregion of a failed pump channelof a lithium processing test loop.

15. Selective Leaching in Molten Salts Molten salts are ionic conductors (like aqueous solutions) and can promote anodic-cathodic electrolytic cells . . . they can be aggressive to metals. ALSO . . . some molten salts (notably fluorides) are “Fluxes” and dissolve surface deposits that would otherwise be protective: dealloying of Cr from Ni-base alloys and stainless steels can occur in the surface layers exposed to molten fluorides; the vacancies in the metal lattice then coalesce to form subsurface voids which agglomerate and grow with increasing time and temperature.

16. (a)(b) (a) microstructure of type 304L SS exposed to LiF-BeF2-ZrF4-ThF4- UF4 (70-23-5-1-1 mole % respectively) for 5700 hours at 688C. (b) microstructure of type 304L SS exposed to LiF-BeF2-ZrF4-ThF4- UF4 (70-23-5-1-1 mole % respectively)for 5724 hours at 685C.

17. EROSION CORROSION

18. EROSION-CORROSION (“Flow-Assisted”or “Flow-Accelerated”Corrosion) An increase in corrosion brought about by a high relative velocity between the corrosive environment and the surface. Removal of the metal may be: • as corrosion product which “spalls off” the surface because of the high fluid shear and bares the metal beneath; • as metal ions, which are swept away by the fluid flow before they can deposit as corrosion product. Remember the distinction between erosion-corrosion and erosion: • erosion is the straightforward wearing away by the mechanical abrasion caused by suspended particles . . . e.g., sand-blasting, erosion of turbine blades by droplets . . . • erosion-corrosionalso involves a corrosive environment . . . the metal undergoes a chemical reaction.

19. Erosion-corrosion produces a distinctive surface finish: grooves, waves, gullies, holes, etc., all oriented with respect to the fluid flow pattern . . . “scalloping”... Erosion-corrosion of stainless alloy pump impeller. Impeller lasted ~ 2 years in oxidizing conditions; after switch to reducing conditions, it lasted ~ 3 weeks! Erosion-corrosion of condenser tube wall.

20. Most metals/alloys are susceptible to erosion-corrosion. Metals that rely on protective surface film for corrosion protection areparticularly vulnerable,e.g.: Al Pb SS CS. Attack occurs when film cannot form because of erosion caused by suspended particles (for example), or when rate of film formation is less than rate of dissolution and transfer to bulk fluid.

21. Erosion-Corrosion found in: - aqueous solutions; - gases; - organic liquids; - liquid metal. If fluid contains suspended solids, erosion-corrosion may be aggravated. Vulnerable equipment is that subjected to high-velocity fluid, to rapid change in direction of fluid, to excessive turbulence . . . viz.equipment in which the contacting fluid has a very thin boundary layer - high mass transfer rates. Vulnerable equipment includes:

22. Surface film effects Protective corrosion-product films important for resistance to erosion-corrosion. Hard, dense, adherent, continuous films give good resistance, provided that they are not brittle and easily removed under stress. Lead sulphate film protects lead against DILUTE H2SO4 under stagnant conditions, but not under rapidly moving conditions. Erosion-corrosion of hard lead by 10% sulphuric acid (velocity 39 ft/sec).

23. pH affects films in erosion-corrosion of low-alloy steel. Scale generally granular Fe3O4 (non-protective). But at pH 6 & pH 10, scale Fe(OH)2/Fe(OH)3 . . . hinders mass transport of oxygen and ionic species. Effect of pH of distilled water on erosion-corrosion of carbon steel at 50C (velocity 39 ft/sec).

24. Dissolved O2 often increases erosion-corrosion . . . e.g. copper alloys in seawater. . . BUT . . . on steels, dissolved O2 will inhibit erosion-corrosion . . . utilized in boiler feedwater systems. Effects of temperature and dissolved O2 on the weight-loss of AISI 304 stainless steel exposed for 800 hours in flowing water at 3.7 m/s.

25. Effect of oxygen dosing on erosion-corrosion and potential of carbon steel in water at 150C, pH at 25oC= 7.8.

26. Good resistance of Ti to erosion-corrosion in: - seawater; - Cl- solutions; - HNO3; and many other environments. Resistance depends on formation and stability of TiO2 films.

27. Chromium imparts resistance to erosion-corrosion to: - steels; - Cu alloys. Such tests have led to the marketing of a new alloy for condenser tubes . . “CA-722” . . . previously “IN-838” . . . with constituents . . . Cu-16Ni-0.4Cr. Effect of chromium additions on seawater impingement-corrosion resistance of copper-nickel alloys. 36-day test with 7.5 m/s jet velocity; seawater temperature: 27C.

28. Velocity Effects N.B. Turbulent flow regime for V < Vc is sometimes called Flow-Assisted Corrosion regime. Schematic showing the effect of flow velocity on erosion-corrosion rate.

29. Relationship between flow velocity, v, and erosion-corrosion rate, w, may be written as . . . w = kva where k and a are constants that depend on the system. DISCUSS: What happens when v = 0 ? How do we express no dependence on velocity? The exponent a varies between . . . 0.3 (laminar flow) and 0.5 (turbulent flow)... occasionally reaching > 1.0 for mass transfer and fluid shear effects. For mechanical removal of oxide films (spalling), the fluid shear stress at the surface is important, and a > 1.0 . . . (may reach 2 - 4).

30. Erosion-Corrosion in Carbon Steel and Low-Alloy Steels N.B. these materials are used extensively in boilers, turbines, feed-water heaters in fossil & nuclear plants. High velocities occur in single-phase flow (water) and two-phase flow (wet steam). Single-phase E-C seen in H.P. feedwater heaters, SG inlets in AGRs, feedwater pumps. Two-phase E-C more widespread . . . steam extraction piping, cross-over piping (HP turbine to moisture separator), steam side of feedwater heaters.

31. Material effects – low-alloy steel . . . Cr additions reduce E-C. Erosion-corrosion loss as a function of time for mild steel and 1 Cr 0.5 Mo steel in water (pH at 25C = 9.05) flowing through an orifice at 130C.

32. Flow dependence (single phase)... Erosion-corrosion rate of carbon steel as a function of flow rate of deoxygenated water through orifice at pH 9.05 and at 149C.

33. Mechanism... for E-C of C.S. in high temperature de-oxygenated water... - magnetite film dissolves reductively Fe3O4 + (3n-4) H2O + 2e 3Fe(OH)n(2-n) + (3n-8)H+ - high mass transfer rates remove soluble Fe II species; - oxide particles eroded from weakened film by fluid shear stress; - metal dissolves to try and maintain film.

34. Mass transfer characteristics correlated by expressions such as... Sh = kRea Scb Sh = Sherwood Number = Re = Re = Reynolds Number Sc = Sc = Schmidt Number Shear stress correlated by …. = ff = friction factor and at high Re, findependent of velocity so

35. Temperature and pH dependence for single-phase E-C of CS . . . Effect of temperature on the exponent of the mass transfer coefficient for the erosion-corrosion of carbon steel in flowing water at various pHs.

36. Prevention of Erosion-Corrosion • design (avoid impingement geometries, high velocity, etc.); • chemistry (e.g., in steam supply systems . . . for CS or low-alloy steel add O2, maintain pH > 9.2, use morpholine rather than NH3); • materials (use Cr-containing steels); • use hard, corrosion-resistant coatings.

37. CAVITATION DAMAGE Similar effect to E-C: mechanical removal of oxide film caused by collapsing vapour bubbles. High-speed pressure oscillations (pumps, etc.) can create shock waves > 60,000 psi. Surface attack often resembles closely-spaced pitting.

38. FRETTING CORROSION Similar to E-C but surface mechanical action provided by wear of another surface . . . generally intermittent, low-amplitude rubbing. Two theories . . . with same overall result . . .

39. Effects in terms of materials COMBINATIONS Fretting resistance of various materials Source: J.R. McDowell, ASTM Special Tech. Pub. No. 144, p. 24, Philadelphia, 1952.

40. Prevention of Fretting Corrosion • lubricate; • avoid relative motion (add packing, etc.); • increase relative motion to reduce attack severity; • select materials (e.g., choose harder component).

41. STRESS CORROSION

42. STRESS CORROSION(“Stress Corrosion Cracking” -SCC) Under tensile stress, and in a suitable environment, some metals and alloys crack . . . usually, SCC noted by absence of significant surface attack . . . occurs in “ductile” materials.

43. “Transgranular” SCC (“TGSCC”) Cross section of stress-corrosion crack in stainless steel.

44. “Intergranular” SCC (“IGSCC”) Intergranular stress corrosion cracking of brass.

45. Two original classic examples of SCC: • “season cracking” of brass; • “caustic embrittlement” of CS;

46. “Season Cracking” Occurs where brass case is crimped onto bullet, i.e., in area of high residual stress. Common in warm, wet environments (e.g., tropics). Ammonia (from decomposition of organic matter, etc.) must be present. Season cracking of German ammunition.

47. “Caustic Embrittlement” Early steam boilers (19th and early 20th century) of riveted carbon steel. Both stationary and locomotive engines often exploded. Examination showed: • cracks or brittle failures around rivet holes; • areas susceptible were cold worked by riveting (i.e., had high residual stresses); • whitish deposits in cracked regions were mostly caustic (i.e., sodium hydroxide from chemical treatment of boiler water); • small leaks at rivets would concentrate NaOH and even dry out to solid. SCC revealed by dye penetrant. Carbon steel plate from a caustic storage tank failed by caustic embrittlement.

48. Factors important in SCC: • environmental composition; • stress; • metal composition and microstructure; • temperature; e.g.,brasses crack in NH3, not in Cl-; SSs crack in Cl-, not in NH3; SSs crack in caustic, not in H2SO4, HNO3, CH3COOH, . . . etc. }necessary

49. STRESS The greater the stress on the material, the quicker it will crack. (N.B. in fabricated components, there are usually RESIDUAL STRESSES from cold working, welding, surface treatment such as grinding or shot peening, etc., as well as APPLIED STRESSES from the service, such as hydrostatic, vapour pressure of contents, bending loads, etc.).

50. Composite curves illustrating the relative stress-corrosion-cracking resistance for commercial stainless steels in boiling 42% magnesium chloride. DISCUSS: how would you obtain such a curve and what does it mean?