Stress Corrosion Cracking Conditions Research Articles

高強度鋼の応力腐食割れき裂発生に及ぼす微小振動応力の影響

Stress corrosion Cracking (SCC) initiation behavior was examined on a high-strength low alloy steel sensitive to hydrogen embrittlement (HE) type SCC under a sustained load with vibratory stresses of small amplitude superimposed in an open circuit and cathodically charged conditions in 3.5%NaCl solution. In an open circuit condition, time to crack initiation under a sustained load with superimposed vibratory stress range greater than 74 MPa is shorter than that of static SCC under a sustained load, resulting in lower 600 ks strength than that of static SCC σSCC. At a stress range Δσ smaller than 74 MPa, however, 600 ks dynamic SCC strength hardly decreases from σSCC. In case of σmax>σSCC SCC cracks are always initiated at corrosion pits formed on specimen surfaces irrespective of superimposed vibratory stress range. On the other hand, at σmax<σSCC and Δσ>74 MPa, the superimposed vibratory stresses promote stress assisted crack-like dissolution preceded by a corrosion pit. Dynamic SCC crack is initiated at the bottom of crack-like portion, thereby reducing the dynamic SCC strength. K*ISCC calculated from the sizes of corrosion pits and crack-like portion is roughly equivalent to KISCC obtained from a long sharp one.When a cathodic potential is applied to specimens, corrosion pits or stress assisted crack-like dissolution preceded by corrosion pits, are no more generated, and no dependence of superimposed vibratory stresses of small amplitude on 600 ks SCC strength is shown. In this situation hydrogen occulusion is much more enhanced than that in an open circuit, and SCC crack is initiated at the interior of the specimen where hydrostatic pressure reaches the maximum.

An ambiguity of the cathodic polarization effect on crack growth during stress corrosion cracking of aluminum alloys

AbstractThe kinetics of crack growth under the conditions of stress corrosion cracking of aluminum alloys in electrolytes with different anion composition and pH values from 0 to 13 has been studied. It is shown that for the majority of alloy‐electrolyte systems anodic polarization accelerates whereas cathodic polarization decelerates crack growth. For “chromic acid – sodium chloride” electrolyte it is shown that cathodic polarization under the conditions of stress corrosion cracking (SCC) has an ambiguous effect on crack growth. The behaviour of A127‐;1 alloy is taken as an illustration of such SCC cracking growth in these electrolytes.

On the Corrosion Behavior of a ferritic 18 Cr‐2 Mo‐steel

AbstractThe investigations carried out with 18 Cr‐2 Mo steel were aimed at its behavior under pitting corrosion, crevice corrosion and stress corrosion cracking conditions. This was done in autoclave laboratory experiments and under experimental heat exchanger conditions in Rhine river water with a chloride content of max. 400 ppm. The test temperatures were 80, 100 and 130°C. Model heat exchangers were fabricated and operated to investigate the influence of filler materials and weld joints between the ferritic 18 Cr‐2 Mo steel and a standard austenitic steel. The possibilities of fabricating tube sheets by applying a weld overlay and using explosive bonding were explored.18 Cr‐2 Mo steel has been shown to be suited for applications in cooling water systems which have a chloride content of 400 ppm. No stress corrosion cracking occurs under such conditions. Tubes with a wall thichness up to 3 mm have sufficient toughness. Tube sheets can be made of boiler plate protected by an explosive cladding or a weld overlay of 18 Cr‐2 Mo. A combination of Type 321 or 304 L and 18 Cr‐2 Mo is possible. Provided 18 Cr‐2 Mo is sufficiently resistant to the product to be cooled, it is an alternative to austenitic CrNi‐(Mo) steels (e.g. AISI 304) when stress corrosion cracking is likely to occur.

Environmentally Enhanced Fatigue Crack Growth of a High-strength Steel Sensitive to Stress Corrosion Cracking

Low-cycle fatigue tests and sustained load tests of a high-strength steel (SNCM8) sensitive to stress corrosion cracking (SCC) have been carried out under various hydrogen contents. The threshold stress intensity facto for cyclic SCC under cyclic stress KFSCC is smaller than the threshold value for static SCC under sustained load KISCC, KFSCC being smaller with an increasing hydrogen content. In a steel tempered from a low temperature no hydrogen embrittlement is caused until diffused hydrogen is concentrated into a plastic zone at crack tips under small hydrogen content, so that the influence of stress waveforms becomes identical with that in a usual corrosion fatigue. Under high hydrogen content cyclic SCC plays a predominant role and a holding period during one cycle most enhances the crack growth. In a steel tempered from a high temperature the corrosion fatigue crack growth characteristics are the same as those in a usual low-strength tough material.

Basal and near- basal hydrides in Ti- 5AI- 2.5Sn

Cleavage cracking of titanium alloys has been reported to occur under conditions of stress corrosion cracking and sustained load cracking and, in such cases, the cleavage plane has often been reported as being near-basal, typically 14 to 16 deg from (000I)α However, the indices of this cleavage plane have not yet been definitely established, nor has the reason for its occurrence. In the present work the Ti-5Al-2.5Sn alloy has been hydrogenated to various levels and three hydride phases are found to occur. The first to precipitate is the y-hydride, occurring at 00123300 ppm H2. At 00123500 ppm H2 a near-basal hydride of unknown crystal structure is formed. There is no unique habit plane. At concentrations in excess of 600 ppm, hydrogen-stabilized stacking faults occur on the basal plane. The significance of these hydride phases is discussed with reference to stress corrosion cracking and sustained load cracking.

Investigation into the Consequences of the Failure of a Turbine-Generator at Hinkley Point ‘A’ Power Station

The further investigation was extensive and is continuing but the present account is limited to what throws light on the Hinkley Point ‘A’ incident and its immediate consequences. There were three main areas of investigation: (1) laboratory and on-site exploration of conditions for stress corrosion cracking of relevant steels; (2) a survey of all relevant rotors to explore the effects of applying a purely fracture mechanics approach in ignorance of the mechanism causing the initial crack or defect; (3) an examination of rotors withdrawn from machines of similar mechanical design and of properties deemed to be in the highest risk category. Interim conclusions have been drawn. They suggest that in certain non-reheat turbines it is possible to generate high chemical concentrations in regions of high stress and thus to promote stress corrosion cracking. The rate of propagation of such cracks is sufficient to produce critical stress intensities in discs of low fracture toughness in a region of high stress concentration.

Corrosion Tunnels in Stainless Steels

Abstract Corrosion tunnels can form in commercially pure austenitic alloys in the absence of chloride ions and stress corrosion cracking conditions.