Nurhan Cansever

The corrosion behaviour of galvanized steel in cooling tower water containing a biocide and a corrosion inhibitor

The corrosion behaviour of galvanized steel in cooling tower water containing a biocide and a corrosion inhibitor was investigated over a 10-month period in a hotel. Planktonic and sessile numbers of sulphate reducing bacteria (SRB) and heterotrophic bacteria were monitored. The corrosion rate was determined by the weight loss method. The corrosion products were analyzed by energy dispersive X-ray spectroscopy and X-ray diffraction. A mineralized, heterogeneous biofilm was observed on the coupons. Although a biocide and a corrosion inhibitor were regularly added to the cooling water, the results showed that microorganisms, such as SRB in the mixed species biofilm, caused corrosion of galvanized steel. It was observed that Zn layers on the test coupons were completely depleted after 3 months. The Fe concentrations in the biofilm showed significant correlations with the weight loss and carbohydrate concentration (respectively, p < 0.01 and p < 0.01).

Inhibition of stress corrosion cracking of aisi 304stainless steel by molybdate ions at elevated temperaturesunder salt crust

In this study, stress corrosion cracking (SCC) occuring under salt crust whichdevelops on the surfaces of hot AISI 304 stainless steels (SS) in neutral solutions wasinvestigated and the inhibition effect of molybdate ions on the SCC of AISI 304 SS under theseconditions was examined. Stress corrosion cracking experiments were carried out under static,Wick test and dynamic, slow strain rate test conditions. The static tests were conducted usingU-bend specimens in chloride and chloride+molybdate containing solutions. The dynamic testswere carried out using slow strain rate (SSR) testing machine under Wick test conditions indistilled water, chloride and chloride+molybdate containing solutions. In the first group ofdynamic tests, the specimens were heated to 100°C and a strain rate of 3.8×10−7 s−1 was used. Inthe second group of tests, the specimen temperature was 130°C at a strain rate of 2.76×10−7 s−1.The specimens strained until fracture. The results of both static and dynamic tests indicated thatthe SCC occured under salt crust in neutral solutions with low chloride ions and cracking couldbe inhibited by the addition of molybdate ions into the solution.

Effects of Ag and Cu ions on the microbial corrosion of 316L stainless steel in the presence of Desulfovibrio sp.

The utilization of Ag and Cu ions to prevent both microbial corrosion and biofilm formation has recently increased. The emphasis of this study lies on the effects of Ag and Cu ions on the microbial corrosion of 316L stainless steel (SS) induced by Desulfovibrio sp. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization were used to analyze the corrosion behavior. The biofilm formation, corrosion products and Ag and Cu ions on the surfaces were investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and elemental mapping. Through circuit modeling, EIS results were used to interpret the physicoelectric interactions between the electrode, biofilm and culture interfaces. EIS results indicated that the metabolic activity of Desulfovibrio sp. accelerated the corrosion rate of SS in both conditions with and without ions. However, due to the retardation in the growth of Desulfovibrio sp. in the presence of Ag and Cu ions, significant decrease in corrosion rate was observed in the culture with the ions. In addition, SEM and EIS analyses revealed that the presence of the ions leads to the formation on the SS of a biofilm with different structure and morphology. Elemental analysis with EDS detected mainly sulfide- and phosphorous-based corrosion products on the surfaces.

New accelerated test for studying the susceptibility of stainless steels to chloride stress corrosion cracking under salt crust

Abstract Stress corrosion cracking (SCC) occurring under the salt crust which develops on the surfaces of hot AISI 304 stainless steels (SS) in neutral NaCl solutions with low chloride ions (1500 ppm) was investigated using a new test technique which incorporated the advantages of slow strain rate test (SSRT) technique applied under Wick test conditions. Type AISI 304 stainless steel specimens were heated at both ends while a glass wool in contact with dilute chloride test solution was wrapped on the middle part of the specimen. Specimens were heated to 100 °C temperature and strained at 1.9 × 10 −5 mms −1 crosshead speed. Another test was done at 130 °C with a 1.38 × 10 −5 mm s −1 crosshead speed. Under both test conditions the specimens had failed by SCC and the cracks had developed under the salt crust on the surfaces of hot stainless steels. The fracture regions of specimens were examined by scanning electron microscope (SEM).

Effect of Mixed-Species Biofilm on Copper Surfaces in Cooling Water System

This study aimed to investigate the formation and effect of a biofilm on copper heat exchangers in full-scale system conditions. A modified Pedersen device with copper coupons was installed in parallel to a heat exchanger system to investigate several physico-chemical parameters, such as bacterial enumeration, carbohydrate content of exopolymeric substances, weight loss of test/control coupons, Cu concentrations, and corrosion products over ten months. Findings of this study showed that planktonic bacterial cells attach to each other and form a mixed-species biofilm on the copper coupon surface even though copper is toxic to a variety of microorganisms. These results also revealed that the mixed-species biofilm has a corrosive effect on copper surfaces used in cooling water systems despite the presence of biocide and the corrosion inhibitor. Additionally, it was demonstrated that a shock-dosed biocide application increased the corrosion rate on copper surface in a real system. Preventing risk of microbiologically influenced corrosion entails appropriate material selection and proper/regular chemical treatment of cooling systems. The current study provides useful insights through the evaluation of corrosion of materials with microbiological techniques.

A comparative study on the dry sliding wear behaviour of nitrocarburised, gas nitrided, fluidised-bed nitrided, and plasma nitrided plastic mould steel

Plastic injection moulds suffer wear due to the relative movements of the mould parts and the polymeric material flow which is often reinforced by hard abrasive fibres or whiskers. Several studies were performed on the wear behaviour of diverse nitrided surfaces; however, comparative studies are needed. The present study aims to compare the dry sliding wear behaviour of nitrocarburised, gas-nitrided, fluidised-bed nitrided, and plasma-nitrided Impax Supreme (equivalent to AISI P20) pre-hardened plastic mould steel. The wear tests were performed against AISI 4140 pins under 100 N normal loads, with the total stroke length of 10 mm, the frequency of 1.5 Hz, and the total sliding distance of 400 m. After tests, worn surfaces and wear debris were microstructurally and chemically characterised and the wear rates were calculated. Results suggested that all samples presented mainly combination of adhesive and abrasive wear during the run-in period of sliding (severe wear), and mainly oxidative wear during the steady-state of (mild wear). Although no significant difference was observed between the treated samples, plasma-nitrided samples presented the lowest wear rate as mean values.

Effect of plasma nitriding time on surface properties of hard chromium electroplated AISI 1010 steel

Abstract Properties of steel can be enhanced by surface treatments such as coating. In some cases, further treatments such as nitriding can also be used in order to get even better results. In order to investigate the properties of nitride layer on hard Cr coated AISI 1010 steel, substrates were electroplated to form hard Cr coatings. Then hard Cr coatings were plasma nitrided at 700 °C for 3 h, 5 h and 7 h and nitride phases on the coatings were investigated by X-ray diffraction analysis. The layer thickness and surface properties of nitride films were investigated by scanning electron microscopy. The hardness and adhesion properties of Cr-N phases were examined using nano indentation and Daimler-Benz Rockwell C adhesion tests. The highest measured hardness was 24.1 GPa and all the three samples exhibited poor adhesion.

Diffusion characteristics of plasma nitrided hard chromium on AISI 1010 steel

Abstract In order to investigate the different Cr-N formation characteristics of plasma nitrided hard Cr coatings, Cr was electrodeposited on AISI 1010 steel and plasma nitrided at 600, 700 and 800 °C for 3 h, 5 h and 7 h, respectively. Phase analyses of resulting Cr-N phases and grain size of Cr layer before and after nitriding process were calculated by X-ray diffraction analysis. Structure of nitride layer and its thickness were analyzed using scanning electron microscopy micrographs. The micrographs indicated that samples consisted of three distinctive layers. In order to distinguish these layers, scanning electron microscopy and energy dispersive spectroscopy (EDX) analysis were used as well as elemental distribution versus depth was plotted. The Cr-N diffusion was investigated by layer thickness measurements, and diffusion coefficient as well as activation energies were calculated.

INVESTIGATION OF THE EFFECT OF BIOFILM FORMATION ON THE BIOCORROSION OF GALVANIZED STEEL BY ELECTROCHEMICAL METHODS

The aim of this study was to investigate the morphology, structure and resistance of the biofilm including mixed bacterial population formed on the galvanized steel surfaces in a model water circulation system by electrochemical methods and to determine the effect of biofilm formation on the corrosion behavior of galvanized steel. The model water circulation system simulating a real cooling tower was run by maintaining uninterrupted supply of water for 3 months. The resistance of biofilm formed on the surface of the galvanized steels, which were removed from the model water circulation system montly, was determined by using electrochemical impedance spectroscopy (EIS). Polarization curves for each coupons were drawn. The biofilm formation and corrosion products were investigated by using scanning electron microscopy (SEM). As a result, it was determined that the structure and resistance of biofilm formed on the galvanized steel surfaces was different in each exposure time and thus the corrosion behavior of galvanized steel surfaces was changed according to time.

NiCr Thin Film Strain Gauges Fabricated on Glass Substrates

Abstract In order to investigate the strain gauge characteristics of NiCr thin films, 500 nm NiCr (80 wt.-% and 20 wt.-%, respectively) thin films were deposited on glass substrates by DC magnetron sputtering. After deposition, NiCr thin films were characterized by using X-Ray diffraction analysis, scanning electron microscope and four-point probe techniques inview of crystallization, phases, film structure and electrical resistivity. After characterization, NiCr thin films were shaped into strain gauges by photo lithography and wet etching techniques. Strain gauges were tested with different loads, and strain values were calculated by comparing the results with commercial NiCr strain gauges with the same surface area. Resistivity change vs. strain was plotted, and the gauge factor of fabricated thin film strain gauges was evaluated as 1.23.