The effects of material formulation and manufacturing process on mechanical and thermal properties of epoxy/clay nanocomposites

A holistic study was conducted to investigate the combined effect of three different pre-mixing processes, namely mechanical mixing, ultrasonication and centrifugation, on mechanical and thermal properties of epoxy/clay nanocomposites reinforced with different platelet-like montmorillonite (MMT) clays (Cloisite Na+, Cloisite 10A, Cloisite 15 or Cloisite 93A) at clay contents of 3–10 wt%. Furthermore, the effect of combined pre-mixing processes and material formulation on clay dispersion and corresponding material properties of resulting composites was investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), flexural and Charpy impact tests, Rockwell hardness tests and differential scanning calorimetry (DSC). A high level of clay agglomeration and partially intercalated/exfoliated clay structures were observed regardless of clay type and content. Epoxy/clay nanocomposites demonstrate an overall noticeable improvement of up to 10 % in the glass transition temperature (Tg) compared to that of neat epoxy, which is interpreted by the inclusion of MMT clays acting as rigid fillers to restrict the chain mobility of epoxy matrices. The impact strength of epoxy/clay nanocomposites was also found to increase by up to 24 % with the addition of 3 wt% Cloisite Na+ clays. However, their flexural strength and hardness diminished when compared to those of neat epoxy, arising from several effects including clay agglomeration, widely distributed microvoids and microcracks as well as weak interfacial bonding between clay particles and epoxy matrices, as confirmed from TEM and SEM results. Overall, it is suggested that an improved technique should be used for the combination of pre-mixing processes in order to achieve the optimal manufacturing condition of uniform clay dispersion and minimal void contents

Effect of BTA on electrochemical corrosion and stress corrosion cracking behaviour of type 304 stainless steel in 1 M HCl

Effect of benzotriazole (BTA) on polarization and stress corrosion cracking (SCC) behaviour of type 304 stainless steel in 1 M HCl was investigated. The anodic polarization curves showed that with BTA additions the anodic polarization kinetics in the active region was not affected, though a reduction in critical current density, i(crit), and passive current density, i(p), was observed. However, BTA was found to influence significantly the cathodic reaction kinetics. SCC results using smooth tensile test specimens showed an increase in time-to-failure, t(f), with BTA additions. Crack growth rate studies using single-edge notched (SEN) specimens showed an increase in threshold stress intensity for SCC, K-ISCC, and a decrease in crack growth rate, da/dt, with BTA additions. While the adsorption isotherms derived from weight toss data followed a Langmuir adsorption isotherm signifying a monolayer adsorption, the adsorption isotherms derived from SCC test data deviated from th is at higher BTA concentrations. The paper discusses the possible reason for this deviation. (C) 1998 Kluwer Academic Publishers

Effect of externally added molybdate on repassivation and stress corrosion cracking of type 304 stainless steel in hydrochloric acid

The effect of externally added molybdate (MoO42-) on the repassivation and SCC behavior of type 304 (UNS S30400) stainless steel (SS) was investigated in 1 M hydrochloric acid (HCl) at open-circuit potential (OCP). With the addition of MoO42-, the time to failure initially decreased hut then gradually increased. From studies using potential-vs-time data, the scratching electrode technique, and electron spectroscopy for chemical analysis (ESCA), the initial decrease in the time to failure at lower MoO42- concentrations was attributed to an increase in the cathodic reaction rate and a consequent increase in anodic dissolution. The increase in the time to failure at higher MoO42- concentrations was attributed to the reduction in anodic dissolution with a strong tendency for MoO42- deposition on the bare metal surface

Passivation and stress corrosion cracking tendency of Ni substituted Mn stainless steels

Stress corrosion cracking (SCC) behaviour of three grades of Ni substituted Mn stainless steels have been studied at room temperature in 1M HCl under constant load at different applied potentials. Repassivation studies have also been carried out using scratching electrode technique. Selective dissolution and film formation similar that of films formed on alpha brass in ammoniacal solutions were found to induce SCC in Mn stainless steels. With decrease in Ni content these alloys were found to be highly susceptible to SCC

Electron spectroscopy for chemical analysis study of corrosion films formed on manganese stainless steels

Films formed on two grades of Mn stainless steels in 1 M hydrochloric acid (HCl) freely exposed to air at different potentials were examined using electron spectroscopy for chemical analysis (ESCA). The Or content of the film, which is related closely to corrosion resistance of the base alloys, was lower within the films formed on Mn stainless steels as compared to a normal type 304 (UNS S30400) stainless steels. The film also contained significant amounts of Mn, Ni, and Cu. It was proposed that the presence of higher amounts of Mn, an electrochemically active element, with Cu resulted in poor passivation behavior of the present high Mn stainless steels