Heat transfer is a critical transport phenomena that can be used effectively in the process industry to achieve efficient energy transfer. But whenever a saline medium comes in contact with a hot metal surface used in heat exchange equipment, it results in corrosion and scale formation or fouling on the surface, thus inhibiting efficient heat transfer. Fouling occurs in conditions where metallic surfaces come in contact with water. Fouling results in decreased operating efficiencies of the system. For instance, a lot of energy is wasted due to biofouling formed in ship hulls. Similarly, pipelines and heat exchangers-using seawater as coolant--suffer from fouling, particularly, biofouling. Biofouling of water-intake structures, equipment, and power-plant piping is a major issue that has a direct effect on the performance of heat exchangers and condensers. Biofouling caused by microbial adhesion usually leads to biocorrosion of metal. This increases safety hazards from conventional and nuclear power plants.

Hence, there is a need to develop water-repellant coatings to decrease the effect of fouling. Many attempts have been made to reduce fouling by coating surfaces with polytetrafluoroethylene (PTFE) owing to its nonstick properties. However, some potential drawbacks of these coatings such as poor thermal conductivity, poor abrasion resistance, and poor adhesion to metal substrate, render them unfit for commercial use. To combat this, electroless nickel-phosporous-PTFE (Ni-P-PTFE) composite coatings were developed. These metal-based nonstick coatings have good mechanical and adhesion strength to the substrate. Their thermal conductivity and antiabrasive properties are also superior to that of ordinary PTFE coatings. However, these coatings were found to peel off during long-term fouling tests, which indicated that the corrosion resistance of these coatings need to be improved.

To address this issue, a research team led by Qi Zhao at the division of mechanical engineering, University of Dundee, UK, have developed autocatalytic-graded nickel-copper-phosporus-polytetrafluoroethylene (Ni-Cu-P-PTFE) composite coatings by the electroless plating technique on stainless steel substrates. Addition of copper to the Ni-P matrix is found to improve the corrosion resistance of the coatings. The group developed Ni-Cu-P-PTFE composite coatings and studied the corrosion rates of the Ni-Cu-P-PTFE composite coatings in sodium chloride (NaCl) solutions and the effects of surface-free energy of these coatings on the adhesion of microbial and calcium sulphate (CaSO4) deposits were studied.

The process involved cleaning the stainless steel samples using alkaline solution at 60 degrees C to 80 degrees C for 10 to 20 minutes and then rinsed with water. Following this, the sample was treated using diluted hydrochloric acid (1 M) for 30 s, and then rinsed with water. A graded Ni-P/Ni-Cu-P/Ni-Cu-P-PTFE coating was prepared on the stainless steel substrate. Using a digital micrometer, the coating thickness was measured. The composite coating was of 22-micrometers thick with the Ni-P/Ni-Cu-P layer measuring 2 micrometers. SEM analysis revealed that the PTFE was uniformly distributed in the Ni-Cu-P matrix. The group also reported that adhesion of bacteria and CaSO4 was minimal when the surface-free energy of the composite coating was in the range of 25 mN/m to 30 mN/m. The corrosion rate of the composite coating was low in comparison with low carbon steel, copper, stainless steel, Ni-P coating, and Ni-P-PTFE coating.

"The incorporation of PTFE nanoparticles into the Ni-Cu-P matrix can take advantage of the different properties of Ni-Cu-P alloy and PTFE. The resulting properties of electroless Ni-Cu-P-PTFE coatings, such as nonstick, higher dry lubricity, lower friction, good wear, and good corrosion resistance, have been used successfully in many industries," Zhao tells Technical Insights.

These cost-effective Ni-Cu-P-PTFE composite coatings with corrosion-resistant properties can be applied to reduce biofouling formation in heat exchangers, pipelines, membrane filtration systems, food processing equipment, and oil pipes. The coatings have been commercialised and the group has patented the technology in Europe (European Patent No: EP03740788.9).

"We plan to apply this technique to produce antifouling heat exchangers," adds Zhao.

Details:

Qi Zhao

Division of Mechanical Engineering,

University of Dundee,

Dundee DD1 4HN, UK

Phone: +44-1382-385651

Fax: +44-1382-385508

E-mail: Q.Zhao@dundee.ac.uk

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