Mechanism of corrosion processes at the tip of a corrosion cracking crack

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 5, pp. 60–65, September–October, 1989.     

Elastic field equations for blunt cracks with reference to stress corrosion cracking

The elastic stress field equations for blunt cracks are derived and presented in a form equivalent to the usual sharp crack tip stress fields. These stress field equations are employed in analyzing a dissolution model for the arrest of stress corrosion cracking by crack tip blunting, which is often observed with the arrest of stress corrosion cracks.

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Zusammenfassung Es wird die Spannungsverteilung vor einem Riss mit einem Kruemmungsradius groesser Null fuer den elastischen Fall abgeleitet and in einer den ueblichen Gleichungen fuer den scharfen Riss aehnlichen Form dargestellt. Diese Spannungsgleichungen werden angewandt, um ein Modell fuer das Anhalten eines Bruchs durch die Abstumpfung der Riss-spitze zu untersuchen. Bei der Spannungskorrosion ist dieses Anhalten eines Bruches haeufig von einer Abstumpfung begleitet.

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Résumé Les équations de champ de tension élastique pour des fissures épointées sont dérivées et présentées de la même façon que celles qui décrivent le champ de tension des pointes de fissures aigues. Ces équations de champ de tension sont employées dans l'analyse d'un modéle de dissolution décrivant l'arrêt du craquement corrosif sous tension obtenu par l'émoussement des pointes de fissures, qu'on observe souvent dans l'arrêt du craquement corrosif sous tension.

     

Mechanism of corrosion cracking of metals deformed in contact with molten lithium

A dislocation mechanism of corrosion cracking of metals in aggressive media is postulated, and the possibility of nucleation and simultaneous propagation of several closely situated cracks is analyzed. Data on the long-time strength of armco iron specimens tested in contact with lithium at 400° C is reported.     

Galvanic elements in corrosion cracks

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 1, pp. 27–31, January–February, 1989.     

Stress corrosion cracking in refrigeration systems

It is generally understood that stress corrosion cracking can occasionally affect the high pressure vessels in ammonia refrigeration systems, but the nature of the problem is often mis-represented in design codes, safety standards and textbooks and ways in which the initiation of stress corrosion can be prevented are ignored. It is also not appreciated that stress corrosion can also affect low pressure vessels and pipework in ammonia systems, and copper pipework in fluorocarbon refrigeration systems.This paper provides an overview of published literature on stress corrosion cracking and then presents some recent case study material illustrating various ways in which stress corrosion has arisen in practice. The conclusions will provide guidance for design engineers and equipment owners on eliminating stress corrosion cracking and on dealing with it if it arises.     

Effective stress intensities in stress corrosion cracking

The phenomena of branching and blunting of stress corrosion cracks are reviewed and their effects demonstrated for a martensitic steel. The stress intensity that a crack can sustain is proportional to the square root of its tip radius, so that blunt cracks require a higher apparent stress intensity. A simple procedure is outlined for converting apparent stress intensities to effective stress intensities, so eliminating anomalous effects due to crack branching and blunting.

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Résumé On passe en revue les phénomènes de ramification et d'arrondissement de l'extrémité des fissures de corrosion sous tension, et on examine leurs effets dans le cas d'un acier martensitique.L'intensité de contrainte qu'une fissure peut supporter est proportionnelle à la racine carrée de son rayon d'entaille, en sorte qu'à des fissures émoussées correspondent une intensité apparente de contrainte plus élevée. On propose une procédure simple permettant de convertir les intensités apparentes de contraintes ou intensité effective, ce qui permet d'éliminer les effets parasites associés à la ramification et à l'arrondissement des fissures.

     

The significance of stress corrosion cracking in corrosion fatigue crack growth studies

A model is proposed to account for interactions between fatigue and stress corrosion crack propagation mechanisms in appropriate corrosion fatigue conditions. Tests on an alloy steel, and both wrought and cast aluminium alloys, are reported. Despite the use of very simple coefficients in the equations derived, encouraging results are obtained.     

Hot corrosion cracking of Nimonic 80A

Experimental investigations have been carried out to study the hot corrosion crack growth behaviour ofN imonic BOA in a combustion gas containing sulphur, vanadium or sodium at 600 and 700°C. The time to rupture has been found to be inversely proportional to the applied stress, and the crack growth rate, da/dt, can be described by the LEFMparameter K, the stress intensity factor. The threshold stress intensity factor, K_th , and the critical stress intensity factor, K_c, have been found to be functions oftemperature and corrodent level. In the case ofsulphur, K_th decreases continuously with increasing sulphur content in the fuel. In the case of vanadium or sodium, there appears to be a critical level ofcorrodent for the worst attack.     

Stress corrosion cracking of maraging steel weldments

Abstract

The stress corrosion characteristics of 18 wt-%Ni (MDN-250) maraging steel and its weldments made under different welding conditions have been investigated. The threshold stress intensity factor K _ISCC in stress corrosion conditions has been determined in 3.5 wt-%NaCl environment for the base metal and weldments. The fractured surfaces were analysed to study the types of fracture during stress corrosion cracking in base and weld metals. Fracture toughness tests were carried out and the results obtained from these tests have been compared with K _ISCC values.