Microbiological deterioration and degradation of synthetic polymeric materials: recent research advances

Stress-corrosion cracking

Correlations between intergranular stress corrosion cracking, grain-boundary microchemistry, and grain-boundary electrochemistry for Al-Zn-Mg-Cu alloys

The AA5083 (Al-4.4Mg-0.7Mn-0.15Cr) alloy is a nonheat-treatable aluminum alloy known for its excellent corrosion resistance. However, it can become susceptible to intergranular stress corrosion cracking (IGSCC) when exposed to temperatures ranging from 50 °C to 200 °C for sufficient lengths of time. This IGSCC is widely believed to be associated with dissolution of the electrochemically active β phase, Al3Mg2, which is precipitated on grain boundaries. Recently, alternative mechanisms have been invoked related to hydrogen effects and/or free Mg segregation or depletion in the grainboundary regions. To establish a baseline for the sensitization effect, constant-extension-rate tests (CERTs) were conducted under open-circuit conditions and under potential control in 3.5 pct NaCl on samples isothermally treated at 150 °C. To aid in interpreting the CERT results, grain-boundary precipitation and solute depletion were characterized by transmission electron microscopy (TEM). Additionally, the electrochemical behavior of the β phase was characterized by anodic polarization of the intermetallic compound synthesized in bulk form. In CERTs under open-circuit conditions, the measured ductility depended strongly on sensitization time, reaching a minimum at 189 hours, followed by a slight increase at longer times. This trend correlated well with the fractional coverage of β phase on grain boundaries, which increased up to 189 hours, where it existed with nearly continuous coverage. At longer times, this film coarsened and became discontinuous. Correspondingly, some resistance to IGSCC was recovered. In polarization experiments, bulk synthesized β phase was found to be spontaneously passive from its corrosion potential (−1.40 VSCE) up to about −0.92 VSCE, where passivity was observed to break down. Sensitized AA5083 samples polarized below the β-phase breakdown potential showed almost no evidence of IGSCC, indicating that a high β dissolution rate is a requirement for IGSCC. Mg-depleted zones were observed along grain boundaries in sensitized alloys, but a clear role for solute depletion in IGSCC could not be defined on the basis of the results developed in this study.

Stress corrosion cracking of sensitized AA5083 (Al-4.5Mg-1.0Mn)

Stress corrosion cracking and hydrogen embrittlement of an Al–Zn–Mg–Cu alloy

The role of magnesium segregation in the intergranular stress corrosion cracking of aluminium alloys

Factors controlling crack velocity in 7000 series aluminium alloys during fatigue in an aggressive environment

Strain-rate effects in the environmentally assisted fracture of a commercial high-strength aluminium alloy (7049)

Specimens of a commercial aluminium-magnesium-zinc alloy (7179-T651) have been tested in tension at low strain rates, either in air or vacuum after pre-exposure to water at 70°C, or in water at various potentials at ambient temperature. Pre-exposed specimens exhibit a reversible embrittlement, producing a decrease in the reduction in area to failure, the flow stress, and the ultimate tensile strength, in excess of that occasioned by testing in moist air. The embrittlement is attributed to hydrogen picked up from the environment and the recovery either to loss of hydrogen (particularly under vacuum) or diffusion to innocuous traps. The potentiostatically controlled low strain rate tests in water at ambient temperature reveal two regimes of embrittlement: one at cathodic potentials capable of generating hydrogen and the other at higher potentials giving rise to anodic dissolution. These processes may both occur at the free corrosion potential in this particular system. As well as confirming the occu...

Reduced ductility of high-strength aluminium alloy during or after exposure to water

Annealed C-Mn steel has been shown to fail by transgranular stress corrosion cracking (SCC) in sodium phosphate solutions over a range of potentials and pH's at room temperature. The obser...

N.J.H. Holroyd

Papers

The role of magnesium segregation in the intergranular stress corrosion cracking of aluminium alloys

Factors controlling crack velocity in 7000 series aluminium alloys during fatigue in an aggressive environment

Strain-rate effects in the environmentally assisted fracture of a commercial high-strength aluminium alloy (7049)

Reduced ductility of high-strength aluminium alloy during or after exposure to water

Stress Corrosion Cracking of C-Mn Steel in Phosphate Solutions