Fabienne Delaunois

Heat treatments for electroless nickel–boron plating on aluminium alloys

Abstract Electroless nickel–boron baths reduced with sodium borohydride can be stabilized with various agents such as thallium nitrate or lead tungstate with no fundamental modification of deposition rates and stability. To improve the mechanical properties of electroless nickel–boron deposits, various heat treatments are applied. At low temperatures, no fundamental changes in the deposit structure are observed, only an improvement of adhesion on aluminium substrate. The values of the Knoop microhardness obtained on these heat-treated deposits are near 600 hk 100 . At higher temperatures, structural changes take place and the nickel–boron deposits crystallize. The microhardness rises until 1050 hk 50 for heat treatments at 350 °C for 4 h. A diffusion layer between the electroless nickel deposit and the aluminium substrate appears at high heat treatment temperatures. The results of scratch tests show that the Ni–B deposits, with or without heat treatments, have good tribological properties under external solicitations. They are hard, wear and abrasion resistant, and also have good adhesion to the aluminium substrate.

Comparison of Various Electroless Nickel Coatings on Steel: Structure, Hardness and Abrasion Resistance

Several electroless nickel deposits, on steel substrate, of varying chemistry were investigated in the as-plated and heat-treated condition: 3 nickel-phosphorous (low, mid and high P) and 2 nickel-boron (nickel-boron (lead) and nickel-boron (thallium)). Samples were characterized by SEM and X-ray diffraction. They were then submitted to Knoops microhardness testing and Taber abrasion tests, with abrasive CS-10 wheels, as well as scratch testing to investigate their mechanical properties and wear resistance. Hardness and wear resistance of all deposits were improved by heat treatment, but the best candidate was the Nickel-boron (lead), with a hardness over 1100 hk50 after heat treatment and a Taber Wear Index of 6. Scratch testing allowed identifying the damage mechanisms of the coated systems.

Corrosion behaviour and biocorrosion of galvanized steel water distribution systems

Galvanized steel tubes are a popular mean for water distribution systems but suffer from corrosion despite their zinc or zinc alloy coatings. First, the quality of hot-dip galvanized (HDG) coatings was studied. Their microstructure, defects, and common types of corrosion were observed. It was shown that many manufactured tubes do not reach European standard (NBN EN 10240), which is the cause of several corrosion problems. The average thickness of zinc layer was found at 41μm against 55μm prescribed by the European standard. However, lack of quality, together with the usual corrosion types known for HDG steel tubes was not sufficient to explain the high corrosion rate (reaching 20μm per year versus 10μm/y for common corrosion types). Electrochemical tests were also performed to understand the corrosion behaviours occurring in galvanized steel tubes. Results have shown that the limiting step was oxygen diffusion, favouring the growth of anaerobic bacteria in steel tubes. EDS analysis was carried out on corroded coatings and has shown the presence of sulphur inside deposits, suggesting the likely bacterial activity. Therefore biocorrosion effects have been investigated. Actually sulphate reducing bacteria (SRB) can reduce sulphate contained in water to hydrogen sulphide (H2S), causing the formation of metal sulphides. Although microbial corrosion is well-known in sea water, it is less investigated in supply water. Thus, an experimental water main was kept in operation for 6months. SRB were detected by BART tests in the test water main.

Accelerated Aging and Portevin-Le Chatelier Effect in AA 2024

AA2024 aluminium alloy is used in aeronautics for various applications. However, the aging treatment (“T4 temper”) at ambient temperature used to bring it to optimal mechanical properties causes several problems. The main one is its duration (5 days) which brings the need for storage of the parts. Moreover temperature control during such a long period is difficult.The aim of this work was to develop an alternative treatment leading to optimized mechanical properties in a shorter time. For this, we investigated the kinetics of the aging process and observed the apparition and disappearance of “Portevin-Le Chatelier (PLC) effect” in AA2024 aluminium alloy.The knowledge of the time at which PLC effect is observed is important in terms of process because shaping may be carried out before the end of the aging process.

Tribological Characterization of Electroless Nickel-Boron Coatings

Nanocrystalline electroless nickel-boron deposits (on steel substrates) were submitted to heat treatment under non-reactive atmosphere or to nitriding to enhance their properties. Two different nitriding processes were used on the samples: one classical industrial process under an ammonia-based atmosphere and a novel, environment friendly treatment under a reduced pressure, nitrogen-based, atmosphere. The mechanical and tribological properties of the as-deposited and treated samples were investigated by various methods including nanoindentation and scratch tests. Their structural properties were also studied. The hardness of the deposits increased from 900 to 1250 hv100 due to optimal crystallization after the heat treatment and was further enhanced up to 1600 hv100 after nitriding. The scratch tests resistance of the coatings was good in both as-deposited conditions as well as after post treatment.

Modeling Approach of the Experiment: Waste Reuse of the Jerada Thermal Power Plant (Morocco)

The main objective of this work is to find a model mathematical statistics which will justify the valorization principle of the FA (Fly Ash) and BA (Bottom Ash), which represent the most remarkable solid waste of the thermal power plant in the eastern region of Morocco (in the Jerada city).