Model Scale Investigation of Blade Root Cavitation Erosion on a Set of Marine Propellers

The present study is focused on the experimental analysis of the erosion caused by cavitation occurring at the blade root for a set of three model scale marine propellers. The experimental method is based on the adoption of soft paint technique together with cavitation observations. Cavitation dynamics and erosion damage patterns are recorded using three standard cameras and one high speed camera. Standard cameras are fixed on the top of test section to continuously monitor the effect of erosion damage on the blade root, the high-speed camera has been placed at different positions to investigate detailed evolution and collapse of bubbles on pressure and suction side of propeller blades. The soft paint test damage patterns have been simultaneously analysed with the high- speed videos, and results showed remarkable agreement between the occurrence of damage on the blades and the bubble collapse of cavitation. The damage pattern and cavities collapse seem to be inversely related with the inception time of damage

Cathodic disbonding tests operating at large cathodic potentials for long periods need current monitoring, pH control and anode isolation

Metallic structures in service in seawater are protected coupling cathodic protection and paints, where the former may induce disbondment of the latter. A preliminary evaluation of the cathodic disbondment risk can be made by cathodic disbondment tests, CDTs. Many CDTs use cathodic potentials as large as E < -1400 mV vs SCE applied up to 90 days. Only two CDT protocols require contemporary anode isolation, current and pH monitoring, without its correction. These three aspects were considered to develop a hybrid CDT; it consisted in polarizing steel panels at -1500 mV vs SCE for 12 weeks. The chemical effects related to the anodic processes were investigated. The observed pH acidic shift was justified by the increasing current demand due to paint damage and brucite precipitation on the panels. The necessity of anode isolating glass to prevent chlorine related chemical attack over the paints, potentially affecting the disbondment result, was verified. In conclusion, current monitoring, pH control and anode isolation are highly suggested to correctly interpret the cathodic disbondment results when CDTs requiring large electronegative potentials are applied for long periods

Influence of thickness on mechanical properties and crack-bridging ability of coatings for concrete

Different variables can influence the crack-bridging ability (CBA) of a coating. These parameters can be related to: (i) the substrate, i.e. the way the crack opens and develops in the concrete; (ii) the substrate/coating interface, i.e. the adhesion of the organic coating to the concrete; (iii) the coating, i.e. its thickness and mechanical properties; (iv) the external conditions, i.e. the temperature in field and the mechanical solicitation of the structure. In the present paper, some experiments were performed to analyse the third point, i.e. to find possible relationships between the variation of the CBA (in situ failure of the coating applied to the concrete substrate) and the mechanical properties (free-film failure) of coatings with thickness. Experimental results suggest that the value of strain at break is a key factor in the variation of CBA of a coating with thickness

Surface treatments for ship hulls-Present situation and trends

Targets of the surface treatments of marine hulls have always been to preserve the structural capacity of the hull envelope and to maintain a smooth and clean external surface, thus minimizing the frictional component of motion resistance. These objectives are pursued since a couple of millennia by fighting the chemical and biological phenomena inducing various forms of degradation in the hull base material and/or in the external surface smoothness. As regards the latter aspect, in particular, a most negative effect is represented by the adhesion of biofouling, jeopardizing the resistance performance of the hull. In the past, solutions to this specific problem were found in biocide-releasing paintings, practice that is nowadays unacceptable because of its high environmental impact. New challenges in the field are represented by the possibility of not only maintaining, but also decreasing, the frictional coefficient intrinsic of the hull surface, while preserving excellent properties against fouling adhesion and minimizing environmental impact. The paper, based on publicly available data, analyses recent trends in the commercially available hull coatings and depicts possible development lines for new types of treatments aimed at reducing frictional resistance

Electrochemical degradation of anionic surfactants

This study was performed to investigate the electrochemical oxidation of anionic surfactants. In particular, a synthetic solution of sodium dodecyl benzene sulfonate and a real car wash wastewater were treated by galvanostatic electrolysis using a Ti\u2013Ru\u2013Sn ternary oxide and a boron-doped diamond (BDD) anode. Measurements of the Chemical Oxygen Demand (COD) and the concentration of the anionic surfactants were used to follow the oxidation. Using the Ti\u2013Ru\u2013Sn ternary oxide anode, the complete removal of COD and sodium dodecyl benzene sulfonate was obtained only in the presence of chloride ions that act as inorganic mediators. The oxidation rate was almost independent of current density and electrolyte flow rate. In the case of BDD the mineralisation of the sodium dodecyl benzene sulfonate was achieved in all experimental conditions due to reaction with hydroxyl radicals electrogenerated on the diamond surface during electrolysis. The COD removal rate increased with increase in electrolyte flow rate, indicating that the oxidation was mass-transfer controlled. Comparison of the results of the two electrodes showed that chlorine mediated oxidation at the Ti\u2013Ru\u2013Sn ternary oxide anode allowed a faster COD removal of both the synthetic solution and real car wash wastewater

Electrochemical technologies for wastewater treatment at pilot plant scale

Process scale-up is a critical, but essential, step in the development of real electrochemical system for oxidation of organic compound. This mini review wants critically analyze the research efforts carried out in the last years about pre-pilot and pilot scale plants, to support the implementation of such technologies in industrial environmental. In the first section are presented the potentiality and the issues related to the anodic oxidation. The second part is dedicated to the electro-Fenton process, while the last is focused on the coupling of the two previous methods. The analysis highlights key factors which are involved in the scaling-up, which are concerning not only the plant size, but involved also their efficiency and economic feasibility