No abstract

Laboratory investigation was performed on the damaged stud bolts of choke valve which has been installed on offshore platforms. The used and keep stud bolt of the same production batch were also examined as a reference. The damaged sample was found badly corroded and parted in a brittle manner. The fracture surface of damaged stud bolt was found has a sign of excessive torsional stress. The chemical composition of keep and used samples proven that stud bolts were in compliance to ASTM A320 grade L7 specification. The coating thickness analysis was conducted on keep and used sample by scanning electron microscope using backscattered signal. A significant depletion occurred on the top coat. Metallographic analysis shows the presence of branched cracking on a damaged stud bolt. It proves that there was a slightly residual stress suffered by the damaged sample. The microstructure for keep and damaged sample were tempered martensite whereas the microstructure of used sample was characterized as fine grained ferriticbainitic microstructure. The laboratory analysis revealed that the root cause of failure was primarily as a result of overtightened of the stud bolts to choke valve during installation. The hardened steel in form of tempered martensite structure is indeed brittle and easily cracked by means of slightly overloaded stress. Another factor was a depletion problem of top coating creates exposure of underneath the metal to corrosive environment.

This paper on optimizing the anodised Al oxide for DNA detection by using Taguchi method. One step anodizing is carry out in the mixture electrolyte using phosphoric and oxalic acid. Control factor for this study is voltage, concentration and time. The corrosion tested to measure polarization resistance. The Taguchi methods will be optimize the anodizing parameter for DNA application. These three samples is then undergo DNA immobilization with 10µM DNA with amine-terminated group and leave it in 4h at RT. The cyclic voltammetry used to detect DNA by measure the immobilization current (Iimmo). From the result, anodized part with DNA immobilized have higher immobilized current thanpolarized part with DNA. Scanning Electron Microscope (SEM) is to observe the surface morphology of the anodized and polarize part. The result show anodized part had more particle and dispersed around than the polarize part.

The corrosion behavior of a AZ80 Magnesium alloy which anodized using a different power source AC and DC up to 30 V in an alkaline silicate solution containing 60 g/L sodium hydroxide, 10 g/L sodium silicate and 40 g/L sodium phosphate has been investigated using linear polarization method. The change of the oxide film morphology both of anodized sample also examined using scanning electron microscopy. Anodization of the AZ80 Mg alloy significantly improves its corrosion resistance for both power source as the potential applied up to 30 V. The Tafel polarization method result demonstrated that corrosion resistant was also increased by voltage applied which also lead to better corrosion resistant. Sample that been anodized with AC power source has more compact surface with pores structure compared to the sample anodized with DC power source. The anodic film is mainly composed of MgO, Mg2SiO4 and Mg3(PO4)2.