Véronique Vitry

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.

Electroless deposition of nickel-boron coatings using low frequency ultrasonic agitation: Effect of ultrasonic frequency on the coatings

&NA; The effect of ultrasound on the properties of Nickel‐Boron (NiB) coatings was investigated. NiB coatings were fabricated by electroless deposition using either ultrasonic or mechanical agitation. The deposition of Ni occurred in an aqueous bath containing a reducible metal salt (nickel chloride), reducing agent (sodium borohydride), complexing agent (ethylenediamine) and stabilizer (lead tungstate). Due to the instability of the borohydride in acidic, neutral and slightly alkaline media, pH was controlled at pH 12 ± 1 in order to avoid destabilizing the bath. Deposition was performed in three different configurations: one with a classical mechanical agitation at 300 rpm and the other two employing ultrasound at a frequency of either 20 or 35 kHz. The microstructures of the electroless coatings were characterized by a combination of optical Microscopy and Scanning Electron Microscope (SEM). The chemistry of the coatings was determined by ICP‐AES (Inductively Coupled Plasma ‐ Atomic Emission Spectrometry) after dissolution in aqua regia. The mechanical properties of the coatings were established by a combination of roughness measurements, Vickers microhardness and pin‐on‐disk tribology tests. Lastly, the corrosion properties were analysed by potentiodynamic polarization. The results showed that low frequency ultrasonic agitation could be used to produce coatings from an alkaline NiB bath and that the thickness of coatings obtained could be increased by over 50% compared to those produced using mechanical agitation. Although ultrasonic agitation produced a smoother coating and some alteration of the deposit morphology was observed, the mechanical and corrosion properties were very similar to those found when using mechanical agitation. HighlightsUltrasound did not bring significant modification of the hardness or wear behaviour.Ultrasonic agitation generates a significant increase of the plating rate.When ultrasonic agitation was employed the coating thickness increased by over 50%.Ultrasonic agitation generates a smoother and less porous morphology.

Chemical, morphological and structural characterisation of electroless duplex NiP/NiB coatings on steel

ABSTRACT Duplex electroless nickel coatings constituted of one layer of nickel-phosphorous and one of nickel-boron are a promising solution to provide combined wear and corrosion resistance to parts. Duplex systems were compared to systems of similar thickness constituted of only one material, in one or two layers. Duplex coatings present intermediate surface texture, but each layer keeps its typical cross-section morphology and structural features, even after heat treatment. The interfaces between the separate layers are sharp in the as-deposited state but not as much after heat treatment. When nickel-boron is deposited first, it influences slightly the grain growth of the subsequent nickel-phosphorous layer, but no influence can be observed when nickel-phosphorous is deposited first.

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.

Mechanical properties of heat-treated duplex electroless nickel coatings

ABSTRACT Electroless nickel coatings present complementary properties: coatings formed using hypophosphite present usually better corrosion resistance while those made with borohydride present better mechanical and wear properties. Duplex coatings, combining both kinds of deposits, are designed to combine features of both nickel–phosphorus (ENP) and nickel–boron (ENB) coatings. In this study, monolayer and bilayer coatings were synthesised on mild steel substrates, in various combinations of ENP and ENB, with a total thickness of 20 µm and heat treated (400°C – 1 h) under non-reactive atmosphere (95% Ar–5% H2) to enhance their mechanical properties. The morphology, roughness, hardness, abrasive wear resistance and scratch behaviour of the coatings were investigated. Samples with ENP as the top layer showed a higher roughness than samples with ENB as the external layer. However, samples with ENB as the top layer provided better wear resistance, even if they presented a lower combined hardness. ENB monolayer showed the best resistance to scratch testing.