Environmentally assisted cracking of aluminium alloys

A short review of the stress corrosion cracking (SCC) behaviour of aluminium alloys is given. Mechanisms of environmentally assisted cracking are outlined. For aluminium alloys, in which stress corrosion cracks propagate predominantly along grain boundaries, anodic dissolution and hydrogen embrittlement have been proposed. Transgranular stress corrosion cracking occurring at severe loading conditions has found particular interest concerning localised corrosion‐deformation interactions. Accelerated test methods for assessing the SCC behaviour are described, including the slow strain rate testing technique and the breaking load method. Results of recent studies on environmentally assisted cracking of aluminium alloys are summarised. Most of the work published in the last two decades has been on aluminium‐lithium based alloys and improved high strength Al‐Zn‐Mg‐Cu alloys in corrosion resistant retrogressed and re‐aged tempers.     

Temporärer Korrosionsschutz von Spannstählen in unverpressten Hüllrohren

Temporary corrosion protection of prestressing steels in non‐injected ducts Prestressing steels may be subjected to corrosive conditions during the manufacturing process at building sites. Due to this a risk of hydrogen induced stress corrosion cracking of the steels may arise. Tests under practical conditions in a prestressed concrete beam were carried out where non‐injected ducts were treated with preheated scavenging air to prove this method being able to protect the prestressing steels against corrosion. The results yielded sufficient corrosion protection by this measure and therefore it may be an interesting alternative in comparison to corrosion protection by film forming agents which contain inhibitors.     

Material selection of safety‐relevant components in indoor swimming pools

Suspended ceilings in indoor swimming pools are safety‐relevant components. As was demonstrated by the collapses of the ceiling of the Uster (CH) indoor swimming pool (1985) and again at Steenwijk (NL, 2001) greater attention has to be paid to selecting suitable materials and inspecting the state of such components. Our findings according to corrosion of metal fastening components of more than 150 indoor swimming pools in Switzerland are reported. The corrosion behaviour of stainless steels and galvanized steels are compared and discussed including newer results from the literature.     

Anion effects on the stress corrosion cracking behaviour of aluminium alloys

The stress corrosion cracking behaviour of plate material of the aluminium alloys 2024‐T351, 8090‐T8171, 7475‐T651, and 7075‐T7351 was investigated performing constant load tests. Short transverse tensile specimens were permanently immersed in aerated aqueous 0.6 M Na₂Cl solutions with additions of Na₂SO₄, NaNO₃, NaHCO₃, NH₄HCO₃, Na₂HPO₄, Na₂SO₃ or Na₂CO₃. The concentration of the added salts was 0.06 M. The applied stress was 100 MPa, except with 7075‐T7351 specimens, which were loaded at 300 MPa. Environment induced failure was not observed in neutral 0.6 M NaCl solution. The various salts added promoted intergranular stress corrosion cracking with the alloys 2024‐T351, 8090‐T8171, and 7475‐T651. Threshold stresses were generally below 100 MPa. For 8090‐T8171 exposed to chloride containing electrolytes with additions of sulfate, hydrogen phosphate, or sulfite, threshold stresses were approximately 100 MPa or higher. Similar results were obtained for 7475‐T651 plate when immersed in chloride‐hydrogen phosphate and chloride‐carbonate solutions. Alloy 7075‐T7351 was resistant against intergranular stress corrosion cracking. Specimens suffered pitting corrosion during immersion in the corrosive environments. Failure observed with 7075‐T7351, in particular when exposed to the chloride‐nitrate solution, was associated with reduction of cross‐sectional area due to pitting and transgranular stress corrosion cracking.     

Zur Schadensverteilung des durch Spannungsriss korrosion geschädigten vergüteten Spannstahls bei Brückenbauwerken

Die Aussagekraft der rechnerischen Untersuchung zur Abschätzung des Gefährdungspotentials infolge Spannungsrisskorrosion hängt entscheidend von den Annahmen zu Schädigungsverlauf und —verteilung des Spannstahls ab. Mit systematischer Untersuchung des aus einem Bauwerk entnommenen Spannstahls konnte festgestellt werden, dass die bisher angenommene Schadenskonzentration an einer Stelle, welche zum Ausfall des gesamten Spannglieds führt, sehr wenig wahrscheinlich ist. Viel wahrscheinlicher ist die drahtweise Schädigung mit unbegrenzter Anzahl der Risse über die gesamte Drahtlänge. Weiterhin ist keine Korrelation zwischen chemischer Zusammensetzung des Spannstahls bzw. zwischen Zusammensetzung des Verpressmörtels und der Versprödung des Spannstahls feststellbar. Weitere Untersuchungen zur Bestätigung der gewonnenen Ergebnisse sind wünschenswert. Damage distribution of hardened and tempered prestressing steel dues to stress corrosion in prestressed concrete bridges. The accuracy of the analysis to estimate the hazard potential due to the stress corrosion depends strongly on the assumption of the damage distribution and the damage progress of the prestressing steel in the constructions. Through a systematic test of the prestressing steel, which is taken from an old bridge, it is shown that the traditional assumption of a damage concentration with the breakdown of the whole tendon is less probable. Instead, the damage of single wires over its total length with unlimited number of cracks is considerably more probable. Furthermore, no correlation between the chemical compositions or mixtures of the grout and the brittleness of the prestressing steel is found. More investigations to check these results obtained in this study are preferable.     

Prediction of the remaining lifetime of stainless steels under conditions of stress corrosion cracking

The prediction of the lifetime of metal structures and equipment under conditions of stress corrosion is very complicated because of the complexity of this process of degradation. Recently a new method, based on the so‐called corrosion elongation curves, has been found, which can be used to predict the time to failure under these conditions. By upgrading of these curves (and thus obtaining Upgraded Corrosion Elongation Curves – UCEC's) it has been possible to obtain a precise definition of the time needed for the initiation of the corrosion crack, and for its stable growth. It is upon this basis that diagrams for the prediction of remaining lifetime (DPRL's) have been developed. DPRL's can also be used to predict the values of various critical parameters which have to be achieved if a stress corrosion crack is to occur.     

Mitigation of chloride driven stress corrosion cracking susceptibility of 316L austenitic stainless steel using plasma sprayed TiO2 coating

The present study proposes a protective TiO₂ coating against chloride driven stress corrosion cracking problem of 316L austenitic stainless steel. To test the performance of the proposed coating, the severe chloride-based boiling magnesium chloride solution at 155 °C was chosen. For experimentation, the constant strain-based U-bend specimens were coated with TiO₂ using atmospheric plasma spray method. The results indicated higher resistance by TiO₂ coated specimens against stress corrosion cracking problem, while the bare specimens experienced severe damage in the boiling magnesium chloride solution under various strain loading configurations. The coating-electrolyte system of TiO₂ coated sample demonstrated over seven times higher resistance, eventually led to reduction in corrosion rate over fifteen times compared to the bare 316L stainless steel in the boiling magnesium chloride solution. This improved performance of the coated 316L stainless steel is attributed to inhibition of outward diffusion of iron-chromium-nickel in the corrosive environment and the high chemical stability of TiO₂.     

FKM‐Richtlinie „Bruchmechanischer Festigkeitsnachweis für Maschinenbauteile”︁

FKM‐Guideline “Fracture Mechanics Proof of Strength for Engineering Components” The German guideline “Fracture Mechanics Proof of Strength for Engineering Components” [1, 2] has been released 2001 as a result of activities sponsored by the Research Committee on Mechanical Engineering (FKM), task group “Component Strength”. The guideline describes basics for the integrity assessment of cracked components subjected to static or cyclic loading and provides a step‐by‐step computational procedure for the use in engineering practice. The guideline was formulated based on a number of national and international reference documents, in particular SINTAP [3], R6 [4], BS 7910 [5] and DVS‐2401 [6], recent research results and some own key aspects. Since 2004 it is also available in English. The procedures and solutions of the guideline are implemented in the computer program FracSafe [7]. The latest 3rd edition of the guideline (2006) includes several new topics. These allow for the consideration of special effects at cyclic loading, mixed mode loading, dynamic (impact) loading, stress corrosion cracking and probabilistic aspects in fracture mechanics calculations. There is a compendium of stress intensity factors and limit load solutions and a compendium of material data. A lot of examples and case studies are included to demonstrate the application of the procedure to engineering problems. This paper gives an overview of the guideline [1].