Stress-corrosion cracking

Role of intermetallic phases in localized corrosion of AA5083

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

The effect of heat input on microstructural zones, hardness and fracture of T-joints in EN S235JR carbon steel was evaluated using the Vickers hardness, SEM, and fractography on impact fractured specimens. The gas metal arc welding (GMAW) process with pulsed metal arc transfer and different welding speeds was performed by an industrial robot using a teaching pendant. Microstructural observations revealed differences in the size and quantity of phases exhibited in the solidified weld bead and recrystallized heat affected zone (HAZ) as well as a refinement of ferrite with different morphologies when a greater heat input was generated. The hardness results indicated greater hardening with greater heat input for the weld bead than the HAZ, which was associated with fine secondary Widmastätten ferrite sideplates growing from columnar grains separated by a larger amount of fine acicular ferrite. At a speed of 35 cm/min, the greater heat input produced a greater increase in volume fraction and width of HAZ with complex microstructures formed by fine-grained ferrite and acicular ferrite. The weld bead reached a greater hardening and volume fraction compared to the HAZ linked to fine acicular ferrite. The hardening of weld bead was confirmed by fractography that showed the presence of a small microvoid morphology I. Guzman-Flores • J.J. Gasca-Dominguez • J. del Prado • L. Rios-Vargas Manufacturera de cigueñales de México (MACIMEX), Centro de Investigación Innovación y Desarrollo Avanzado, Km. 38.5 Carretera la Marquesa, Tenango del Valle, Estado de México 52300, México e-mail: Isidro.Guzman@macimex.com; JuanJose.Gasca@macimex.com; Joaquin.DelPrado@macimex.com; Leopoldo.Rios@macimex.com B. Vargas-Arista (*) División de Estudios de Posgrado e Investigación, Instituto Tecnológico de Tlalnepantla, Av. Instituto Tecnológico s/n, Col. La Comunidad, Tlalnepantla de Baz, Estado de México 54070, México e-mail: benjaminvar@yahoo.com.mx E. Garfias-Garcia División de CBI, Departamento de Materiales, Área de Ingeniería de Materiales, UAM –Azcapotzalco Av. San Pablo No. 180, Col. Reynosa Tamaulipas, Azcapotzalco 02200, México e-mail: D.F.elgg@correo.azc.uam.mx

Materials Characterization I

Al–Mg alloy AA5083 is a sheet and plate alloy used mainly for marine application as well as for structural components in transportation and military applications. The strength is derived from solid solution strengthening and strain hardening. The properties of as-received and sensitized samples of AA5083-H116 were investigated using microhardness measurements, tensile testing, optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Nitric Acid Mass Loss Test (NAMLT). The results show that both chemical and mechanical properties of the alloy decreased with increasing sensitization temperature and time. The deterioration in chemical property, which was measured in terms of the level of susceptibility to Intergranular Corrosion (IGC), is attributed to grain boundary precipitation of magnesium-rich particles. The loss in tensile and hardness properties is attributed to softening caused partly by decrease in Mg solute solid solution concentration with increasing sensitization time and temperature and partly by recrystallization at elevated temperatures.

Effect of sensitization heat treatment on properties of Al–Mg alloy AA5083-H116

The corrosion and stress corrosion cracking (SCC) susceptibility of an Al-Mg alloy, AA5083, has been shown to depend on the precipitation of the Mg-rich β phase, (Al3Mg2), but not the enrichment of elemental Mg at grain boundaries to an enrichment ratio of 1.4. These results were determined by measuring the progress of Mg enrichment at grain boundaries, for increasing thermal-treatment times, using auger electron spectroscopy (AES) of grain boundaries exposed by fracture within the spectrometer and by analytical electron microscopy (AEM) of thin foils. The progress of the β phase precipitation was followed by AEM and scanning electron microscopy (SEM), for the same thermal-treatment times. The lack of a Mg-segregation effect on SCC was demonstrated by results obtained with X-ray photoelectron spectroscopy (XPS) analysis of Mg-implanted Al following in-situ electrochemical tests and SCC tests, while the dominance of β phase precipitation was demonstrated by electrochemical analysis and SCC testing. Crack-growth tests of alloy AA5083 demonstrated faster cracking at potentials anodic to the open circuit potential (OCP) with no increase at potentials cathodic to the OCP.

Role of Mg in the stress corrosion cracking of an Al-Mg alloy

Use of the Devanathan–Stachurski cell to measure hydrogen permeation in aluminum alloys

High temperature pressure Ag/AgCl electrodes resistant to the contaminating effects of H/sub 2/ and H/sub 2/S were described in 1979. But the external reference electrode suffers from a thermal liquid junction potential (TLJP) and the development of the Soret effect (thermo-diffusion of the electrolyte). This paper describes a design to improve electrode life. A silvered-tube replaces the silver wire electrode. A liquid chromatograph minipump is used to slowly inject KC1 solution into the electrode to maintain a ''front'' of fresh solution. This prevents ageing effects, and the Soret effect. Construction specifications are given for: corrosion resistant tubing, electrical gland end, zirconium yarn, titanium tubing, polyimide collar, PTFE tubing, minipumps for chromatograph, zirconia plug, and silver chloride.

Technical Note: A Long-Lived External Ag/AgCl Reference Electrode for Use in High Temperature/Pressure Environments

The effect of P and S on the intergranular corrosion and stress corrosion of Ni has been evaluated with corrosion tests, crack tip corrosion modeling and stress corrosion tests, and modeling. Tests were conducted in I N H2SO4 at 25 °C over anodic potentials of +0.3 V to 2.0 V (SCE). Corrosion tests were conducted on Ni + P and Ni + S alloys and Ni samples ion implanted with P and S. The corrosion results showed that both P and S reduce passivity in Ni and enhance the corrosion rate but that P is oxidized and dissolves in the electrolyte while S remains on the surface. This difference in surface behavior of P and S was used to explain the difference in intergranular corrosion (IGC) and inter-granular stress corrosion cracking (IGSCC) behavior of Ni + P and Ni + S. The active crevice and crack walls with Ni + S is thought to reduce the concentration gradient and hence the transport of Ni+2 from tip to mouth which in turn reduces the crack tip corrosion rate. By comparison of the geometry, potential dependence and rates of IGC and IGSCC of Ni + P it was concluded that IGSCC was not controlled solely by anodic dissolution but that a mechanical contribution was involved. Strain assisted corrosion was ruled out while a corrosion assisted intergranular fracture process was shown to be feasible.

M. J. Danielson

Papers

Role of Mg in the stress corrosion cracking of an Al-Mg alloy

Use of the Devanathan–Stachurski cell to measure hydrogen permeation in aluminum alloys

Technical Note: A Long-Lived External Ag/AgCl Reference Electrode for Use in High Temperature/Pressure Environments

Role of segregated P and S in intergranular stress corrosion cracking of Ni

The effects of sulfur on the dissolution of nickel