RELACIÓN DE SOPORTE DE CALIFORNIA (CBR)

Fatigue crack growth data from Thorpe et al [ 1983] suggests that very negative cathodic potentials may have a detrimental effect on BS4360:355D in artificial sea water. This data has been put into a schematic by Procter [ 1986] and is shown in Fig.4.20. The data shown was obtained at high R ratio and at 0.1 Hz. The effects of cathodic potential are less pronounced at lower R ratio and higher frequencies. Tests on API-X65 steel also show that cathodic protection and particularly cathodic overprotection can result in accelerated crack growth rates when compared to free corrosion and air data.

Detailed fractographic studies have revealed that this acceleration at high cathodic potentials is the direct result of hydrogen embrittlement [Procter, 1986]. Fractographs of specimens tested in air reveal that the fracture surface is generated by predominantly ductile fracture modes (tearing and microvoid coalescence) and very few brittle, intergragnular facets at low AK values can be seen. Under free corrosion conditions, more intergranular facets are observed, and brittle, transgranular cleavage facets make a significant contribution to crack propagation. As applied cathodic potential becomes more negative the

contribution of the ductile modes of fracture decreases and brittle, hydrogen-induced fracture modes (intergranular and transgranular cleavage) dominate the fracture surface.

The implication is that sufficient hydrogen is generated within the crack enclave to cause embrittlement at the crack tip. The role of low frequency cyclic straining at the crack tip is to permit hydrogen entry, and to assist hydrogen diffusion to regions favourable to crack nucléation. Lower potentials and frquencies enhance hydrogen entry and therefore give rise to accelerated crack growth rates. This would be in line with hydrogen permeation currents measured by Nielsen et Maahn [1987] already discussed in section 2.2 (Fig.4.3). The role of calcareous deposits was also emphasised in that section.

Calcareous deposition, however, not only prevents hydrogen entry into the material, but also plays a major role in the electrochemical behaviour of a corrosion fatigue system. A number of studies attempting to establish the crack tip potential in deleterious environments have been undertaken (eg Verillyea et Tedman [1970] and Ateya et Pickering [1975]). These have shown that potential drops at the crack tip could be of the order of lOOmV for cracks up to 10mm deep, and even larger where hydrogen bubbles are trapped inside the crack. The variations in current distribution along a specimen containing a crack and being subjected to cathodic potential can be visualised in a diagram from Congleton and Craig’s review [1982] (Fig.4.21). In this electrical analogy the current flow between a block of metal with a slot and a platinum counter electrode is measured. It can be seen that the current density on the external surface is high and uniform but it is drastically reduced at the crack tip region. As a result of this, one would expect large potential drops along the crack depth of a cathodically protected specimen, but this is not observed experimentally. The reason for this is that passivating films are known to form on the surface of the material being protected, thus reducing the current density required to protect the metal. Passivating films keep forming throughout the surface up to a point when sufficient current is available at the crack surface for passivating films to form there too. In this way the whole specimen may end up being cathodically protected (Fig.4.22).

It follows from this that after crack propagation, new, unpassivated surfaces are generated and unless sufficient current density is available the electrochemical conditions at the crack tip will approach those in free corrosion. It is obvious therefore that different cathodic protection set-ups with varying current output will have a different time response. Potential drop measurements have been obtained by Congleton et Craig [1982] and by Hodgkiess [1990].

4.3 Summary

The various variables involved in corrosion fatigue have been studied and a number of general conclusions have been reached. These can be summarised as followsiHigh CP levels tend to produce short initiation lives when subjected to high stress ranges (Fig.4.8); crack growth rates tend to be faster with more negative CP (Fig.4.20); and the strain rates seem to have an effect on FCGR’s, especially at the microscopical level where deposition and rupture of passivating films occurs.

4.4 References

Ateya BG, Pickering HW,J Electrochem Soc, 122, 1018-1026, 1975

Austin IM, Walker EF, International Conference on the Effect of Environment on Fatigue, Publ 4, ppl-10, 1977

Barsom JM, "Effect of Cyclic Stress Form on Corrosion Fatigue Crack Propagation Below Kiscc in High Strength Steel", Corrosion Fatigue: Chemistry, Mechanics, and Microstructure, Conference held at the University of Connecticut, June 1971, edited by O Devereux, AJ McEvily and RW Staehle, NACE handbook 2, 1973

Bignonnet A, Brazy JL, Vallet C, Barrere F, "The Influence of Electrochemical Parameters on the Corrosion Fatigue of Notches of a Structural Steel", Steel in Marine Structures, Paris, published by Elsevier, 1987

Congleton J, Craig IH, "Corrosion Fatigue", Corrosion Processes, edited by RN Parkins, Applied Science publishers, 1982

Duquette DJ, "A Review of Aqueous Corrosion Fatigue", Corrosion Fatigue: Chemistry, Mechanics, and Microstructure, Conference held at the University of Connecticut, June 1971, edited by O Devereux, AJ McEvily and RW Staehle, NACE handbook 2, 1973

Evans UR, Corrosion and Oxidation of Metals, Edward Arnold, London Cited by Congleton J, Craig IH, "Corrosion Fatigue", Corrosion Processes, edited by RN Parkins, Applied Science publishers, 1982

Gerberich WW, Moody NR, "A Review of Fatigue Fracture Topology Effects on Threshold and Growth Mechanics", Fatigue Mechanisms, edited by JT Fong, ASTM STP 675, pp292-341,1979

Gibala R, "Hydrogen-Defect Interactions in Iron-Base Alloys", Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, Conference held at Unieux-Firminy, France, June 1973, edited by RW Staehle, J Hochmann, R D McCright, and JE Slater, NACE Handbook 5, 1977

Gilbert PT, Metallurgical Reviews, 1, pp379-417,1956 Gough HJ, J Inst of Metals, Vol 39, pl7, 1932

Gould, Engineering, 141,495, 1936.

Cited by Duquette DJ, "A Review of Aqueous Corrosion Fatigue", Corrosion Fatigue: Chemistry, Mechanics, and Microstructure, Conference held at the University of Connecticut, June 1971, edited by O Devereux, AJ McEvily and RW Staehle, NACE handbook 2, 1973,

Habashi M, Philipponneau G, Widawski S, and Galland J, "Interactions Between Fatigue Crach Growth Rate and Kinetics of Magnesium Calcium Deposit Formation at Crack Tip of Mild Steel Cathodically Polarized in Sea Water", International Conference of Fracture, New Delhi, 1984

Hodgkiess T, "Corrosion Fatigue of Structural Steels - Studies of Crack-Tip Electrochemistry", Cohesive Fatigue Programme 1987-89, Edited by WD Dover and S Dharmavasan, University College London, 1990.

McAdam Jr DJ,Cong Inter Lessai Matériaux, Amsterdam, pp305-358, 1928. cited in [Congleton et Craig, 1982]

Mughrabi H, Ackermann F, Herz K, "Persistent Slipbands in Fatigued FCC and BCC Metals", Fatigue Mechanisms, edited by JT Fong, ASTM STP 675, pp292-341,1979

Nielsen HP and Maahn E, "Environmental Effects in Fatigue Crack Initiation", Steel in Marine Structures, Paris, published by Elsevier, 1987

Procter RPM, UMIST, Manchester, private communication, 1990

Procter RPM, "Detrimental Effects of Cathodic Protection: Embrittlement and Cracking Phenomena", Cathodic Protection: Theory and Practice, edited by V Ashworth and CJL Booker, published by Ellis Horwood Ltd, 1986

Radd FJ, Crowder LH, Wolfe LH, Corrosion, 16, 415, 1960

Cited by Duquette DJ, "A Review of Aqueous Corrosion Fatigue", Corrosion Fatigue: Chemistry, Mechanics, and Microstructure, Conference held at the University of Connecticut, June 1971, edited by O Devereux, AJ McEvily and RW Staehle, NACE handbook 2, 1973,

Scully JR, Moran PJ, "Influence of Strain on Hydrogen Assisted Cracking of Cathodically Polarized High-Strength Steel", Environmentally Assisted Cracking: Science and Engineering, ASTM STP 1049, edited by WB Lisagor and BN Leis, pp5-29, 1990

Taira S, Tanaka K, Hoshina M, "Grain Size Effect on Crack Nucléation and Growth in Long-Life Fatigue of Low-Carbon Steel", Fatigue Mechanisms, edited by JT Fong, ASTM STP 675, pp292-341, 1979

Thorpe TW, Scott PM, Ranee A, Silvester D, "Corrosion Fatigue of BS4360:50D Structural Steel in Sea Water", Int J Fatigue, Vol.5, No.3, July 1983

Vermiliyea DA, Tedman CS, J Electrochem Soc, 117, 437-439,1970 Cited by Congleton et Craig [1982].

Vosikovsky O, Bell R, Bums DJ, Mohaupt UH, "Effects of Cathodic Protection and Thickness on Corrosion Fatigue Life of Welded Plate T Joints", Steel in Marine Structures, Paris, published by Elsevier, 1987

Vosikovsky O, "Effects of Mechanical and Environmental Variables on Fatigue Crack Growth Rates in Steel. A Summary of Work Done atCANMET", Canadian Metallurgical Quarterly, Vol 19, pp87-97,1980.

Wei RP, "Environmentally Assisted Fatigue Crack Growth", Advances in Fatigue Science and Technology, edited by C Moura Branco and L Guerra Rosa, Kluwer Academic Publishers, pp221-252,1989

Wei RP, "Fracture Mechanics and Corrosion Fatigue", Corrosion Fatigue: Mechanics, Metallurgy, Electrochemistry and Engineering, edited by TW Crooker and BN Leis, ASTM STP 801, pp5-25,1983

Wei RP, "On Understanding Environment Enhanced Fatigue Crack Growth - a Fundamental Approach", Fatigue Mechanisms, edited by JT Fong, ASTM STP 675, pp816-840,1979

see

P A T t G U E

( b ) L o g ( K ) L o g ( A K)

TeF seF TeF&seF

a g g r e s s i v e a g g r e s s i v e i n e r t i n e r t i n e r t Cd) ( e ) L o g ( AK) L o g ( AK) L o g ( AK)

Fig. 4.1 Schematic Illustrations of Basic Types of Corrosion