The crack growth resistance behaviour of aluminium alloy 2024-T3 at slow strain rates after exposure to standard corrosive environments

SYNOPSIS The study investigates the effect of prior corrosive exposure on crack growth resistance behaviour of thin sheet (3 mm thick) aluminium alloy 2024-T3 at slow strain rates. Compact tension specimens were exposed to standard corrosive environments that simulate accelerated atmospheric corrosion attack. TWo corrosive environments were considered - an exfoliation corrosion (EXCO) solution and a 3.5 wt% sodium chloride solution. The unloading compliance R-curves of the two-hour EXCO-exposed specimens revealed a significant degradation of approximately 11% in the crack growth resistance behaviour (Kcevalues) compared to the baseline (air-exposed) values. Furthermore, secondary intergranular crack formation was also revealed in the plastic zone ahead of, and adjacent to, the crack tip of these specimens; which formed during the crack growth resistance loading. It is postulated that the observed degradation of the Kce values of the EXCO-exposed material is due to hydrogen embrittlement since the exposure times for the EXCO evaluation were limited to ensure that uniform corrosion dominated; that is, significant penetration of corrosion damage and pitting due to localized corrosive attack did not occur. The sodium chloride-exposed specimens revealed a similar degradation (13%) after 24 hours exposure. However, slight intergranular corrosive attack and isolated pitting were observed on the exposed surfaces prior to crack growth resistance loading, resulting in notch effects that could assist in crack growth. Pitting and intergranular corrosion were, however, not observed at the pre-crack tip. The relative contributions of the notch effects and the hydrogen embrittlement during the degradation of the KR performance are, therefore, unclear. Keywords: corrosion, crack growth, aluminium alloy 2024-T3.

Corrosion Resistance of Boronized, Aluminized, and Chromized Thermal Diffusion-Coated Steels in Simulated High-Temperature Recovery Boiler Conditions

In this study, the high-temperature molten salt corrosion resistance of bare steels and steels with protective coatings, fabricated by thermal diffusion processes (boronizing, aluminizing and chromizing), were investigated and compared. Surface engineering through thermal diffusion can be used to fabricate protective coatings against corrosion, while alleviating issues around possible cracking and spallation that is typical for conventional thermal-sprayed coatings. In this regard, samples of low carbon steel and 316 stainless steel substrates were boronized, chromized, and aluminized through a proprietary thermal diffusion process, while some of the samples were further coated with additional thin oxide and non-oxide layers to create new surface architectures. In order to simulate the actual corrosion conditions in recovery boilers (e.g., from black liquor combustion), the surfaces of the samples sprayed with a modeling salt solution, were exposed to low-temperature (220 ∘C) and high-temperature (600 ∘C) environments. According to microstructural and X-ray diffraction (XRD) studies and results of hardness determination, the coatings with multilayered architectures, with and without additional oxide layers, showed successful resistance to corrosive attack over bare steels. In particular, the samples with boronized and chromized coatings successfully withstood low-temperature corrosive attack, and the samples with aluminized coatings successfully resisted both low- and high-temperature molten salt corrosive attacks. The results of this study conducted for the first time for the thermal diffusion coatings suggest that these coatings with the obtained architectures may be suitable for surface engineering of large-sized steel components and tubing required for recovery boilers and other production units for pulp and paper processing and power generation.

Weld Overlay of Highly Corrosion Resistant Nickel Chromium Molybdenum Alloys, UNS N06059, on Low Alloy Equipment Operating at High Temperature

Corrosion conditions in the Chemical and Oil and Gas Industries are such that equipment must withstand pressure and corrosion at high temperatures. Due to the fact that in many cases only the internal surface is exposed to corrosive attack, and so only that surface needs to be protected, savings on material costs can be achieved by taking advantage of the weld overlay technique. Weld overlay, through the choice of alloy, is selected to resist to the corrosive attack. Parent material can be selected based on the required strength at temperature. In recent years, highly corrosion resistant Nickel Chromium-Molybdenum Alloys have been developed to operate in the most severe corrosive environments. One of the recent developments in this family of materials, is Alloy 59 UNS N06059 that is replacing Hastelloy C-2000 UNS N06200 and Hastelloy C-22 UNS N06022. Nickel-Chromium-Molybdenum alloys are the most versatile nickel alloys, because they contain molybdenum, which protects against corrosion under reducing conditions, and chromium which protects against corrosion under oxidizing conditions. Wrought products produced from these alloys are processed to obtain a homogeneous austenitic grain structure. Weld overlay or weld deposit, due to the fundamental differences in processing compared to base material, result in a more heterogeneous metallurgical grain structure that in general shows however a good level of corrosion resistance. Weld or Weld overlay features are highly dependent on electric parameters, filler metal, technique, base material temperature, etc., and for this reason the general settings of the welding process need to be finalized and tested, with the intent of optimizing all these parameters in order to allow the best corrosion results. A research activity has been carried out by ALFA LAVAL OLMI to define a processes and parameters’ selection in order to obtain a weld deposit with a behavior as close as possible to wrought Alloy 59 base material.

Influencia del tipo de sinterizado en el comportamiento anticorrosivo de recubrimientos sol-gel

Este trabajo muestra un estudio electroquímico de recubrimientos SiO2 TiO2 ZrO2 obtenidos por el método sol-gel, sinterizados vía plasma y vía convencional. Los recubrimientos se conformaron a partir de Si(OC2H5)4, Ti(OBu)4 y Zr(OC3H7)4, se depositaron sobre sustratos de acero ANSI/304 mediante dip-coating en monocapa y bicapa. El proceso de densificado vía plasma se realizó a temperaturas del sustrato de 250 ºC, 300 ºC y 330 ºC y el convencional a 400 ºC y a una velocidad de calentamiento de 2 ºC/min. El comportamiento anticorrosivo se estudió mediante las técnicas de espectroscopía de impedancia electroquímica (EIS) y Tafel. Se observó que el proceso de sinterización vía plasma, mejora la resistencia a la corrosión con respecto al método convencional.Palabras Clave: sol-gel; corrosión; plasma; EIS;Tafel Abstract This work shows a comparative electrochemical study among the anticorrosive properties of coatings SiO2 TiO2 ZrO2 obtained by the method sol-gel and plasma sintering process and conventional. The coatings conformed to starting from Si(OC2 H5 )4 , Ti(OBu)4 and Zr(OC3 H7 )4 . These were deposited on steel substrate 304 by means of dip-coating in mono-layer and bi-layer. The plasma sintering process was carried out to temperatures of the substrate of 250 ºC, 300 ºC and 330 ºC and the conventional to 400 ºC and a speed of heating of 2 ºC/min. the anticorrosive behavior was studied by means of the techniques of electrochemical impedance spectroscopy (EIS) and Tafel. It was observed that the plasma sintering process, it improves the resistance to the corrosive attack to in regard to the conventional method.Keywords: sol-gel, corrosion, plasma, EIS, Tafel