Crack-free Chromium Research Articles

Metallization of polypropylene substrates through surface functionalization and physical vapor deposition of chromium coatings

Physical vapor deposition (PVD) of chromium coatings represents an interesting alternative to electroplating to obtain metallized plastic parts for a variety of manufacturing sectors. This process is however hampered by the limited adhesion of the metallic layer on the underlying polymeric substrates. Within this context, PVD metallization of polypropylene substrates has so far been particularly challenging given the low surface energy and poor wettability of this commodity plastics. To address this issue, a comprehensive approach is proposed here to enable the production of PVD-metallized PP substrates with excellent and stable interlayer adhesion. The adhesive properties of the PP substrate are modified by a selective UV-induced photografting process in which vinyl-, glycidyl- or 2-hydroxyethyl methacrylate species are covalently attached to the PP surface, as confirmed by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and surface wettability measurements. Based on the chemistry of the photografted functionality, different resin systems are selected and investigated for use as primers to enable subsequent chromium deposition, including acrylic, epoxy and polyurethane systems. The effect of the photografting process on the adhesion between PP and the primers is systematically assessed by means of pull-off tests, highlighting a significant improvement of the adhesion force after surface functionalization with appropriate grafting agents. Finally, functionalized PP substrates are chromated through PVD, obtaining homogeneous and crack-free chromium surfaces upon judicious selection of suitable primer-grafting agent combinations. This also led to outstanding interlayer adhesion and micromechanical performance. This work provides the first demonstration of chromium-metallization of PP substrates via PVD for metallized plastic components with excellent surface characteristics, and paves the path for the design of mechanically durable and aesthetically-compliant PVD-metallized PP components.

Microstructural evolution and wear properties of chromium carbide coating formed by thermo-reactive diffusion (TRD) process on a cold-work tool steel

In the current study, the possibility of forming a chromium carbide layer on the surface of an AISI W1 cold work tool steel was investigated. The coatings were fabricated through a thermo-reactive diffusion (TRD) treatment by the molten salt bath at 800, 900, and 1000 °C for 4, 5, and 6 h using chromium oxide powder. The structural and mechanical properties of the coatings formed at the different conditions (temperature and immersion time) were compared together. For this purpose, the coating layers were characterized by field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectrometer (EDS) as well as X-ray diffraction (XRD) method. Moreover, the hardness and friction coefficient (COF) of the coated samples were examined through micro-indentation and ball-on disk tests, respectively. It was determined that at 800 °C, just chromium carbide nuclei are formed, and the temperature is insufficient to produce a complete chromium carbide layer on the substrate. XRD results of the coating produced at this temperature showed that the main phase is Fe–Cr solid solution. However, compact, adherent, uniform, and crack-free chromium carbide coatings with thickness ranging from 5.14 to 18.06μm were successfully obtained on the substrate at 900 and 1000 °C, mainly consisting of Cr7C3, Cr23C6, and Cr3C2. The XRD results showed that the possibility of forming chromium carbide phases increases with increasing temperature and immersion time. The hardness of the coatings was in the range 771 to 1600 HV, depending on the process condition. The results demonstrated that increasing the temperature and immersion time of the TRD process increases the thickness and hardness of coatings, resulting in low friction coefficient values and improvement in the wear resistance up to 6 times compared to the untreated AISI W1 sample.

A New Method for Electroplating of Crack-Free Chromium Coatings

In this study, different amounts of MoS2 particles and an anionic surfactant were added to the conventional chromium electroplating bath in order to electrodeposite crack-free chromium coatings and the structure, morphology, tribology and corrosion behavior of the deposited coatings were investigated using X-ray diffraction spectroscopy, scanning electron microscopy, pin on disk wear test method, dynamic polarization, and electrochemical impedance spectroscopy techniques, respectively. The results showed that the incorporation of MoS2 particle into the electrodeposited Cr coatings is impossible and no chromium coating can be electroplated from baths containing more than 5 g/L MoS2 particles. In addition, as the MoS2 concentration in the bath increases up to 1 g/L the corrosion and wear resistance of the deposited coatings increases. Moreover, those coatings that were electroplated from the baths containing more than 1 g/L exhibited less performance regarding their corrosion and wear behavior.

Experimental Research on Hard and Crack-Free Electrodeposited Chromium Coatings

In order to eliminate cracks in the chromium coating, a novel method named flexible extrusion assisted chromium electroplating was proposed. The bright and crack-free chromium coatings were electrodeposited by using the perturbation and extrusion of hard and insulating particles. The prepared deposits were characterized by scanning electron microscope (SEM) and three-dimensional topography. The testing results showed that the surface was very smooth and there was no micro cracks in the coating. Micro hardness of the electrodeposited layer was also tested. It was confirmed that the chromium coating electrodeposited with rotating cathode in hard particles had high micro hardness as to 850HV and the micro hardness could be controlled by the process parameters. The rule of the micro hardness was concluded by analyzing current density and rotating speed. In addition the results of salt spray test and electrochemical polarization curve showed that the coating deposited by new method had higher corrosion resistance than that by traditional method.

Chemometrics applied to functional chromium electroplating by pulse plating techniques

Electrodeposited chromium coatings offer high levels of mechanical and tribological properties that justify their use in many industrial and engineering applications. However, one problem of these functional coatings is their limited corrosion resistance and the low current efficiency of their electrodeposition process. Reverse pulse plating can lead to an improvement in the characteristics of chromium deposits, but the high number of variables involved and the interactions among them make the selection of operating parameters difficult. In this paper, a chemometric approach was used in order to design the experiments and study the effect of pulse plating parameters on the properties of chromium coatings. Multivariate analysis was proposed to establish a relationship among structural and physical characteristics of chromium deposits and to determine the pulse parameters required to achieve certain properties. The results indicated that by reverse pulse electrodeposition, it is possible to obtain crack free chromium coatings, with similar hardness values as those obtained from direct current electrodeposited coatings, but with an improved corrosion resistance.

A Comparison between the Corrosion Resistances of High Velocity Oxy Fuel (HVOF) Sprayed Coatings and Hard Chromium Coatings

HVOF-sprayed coatings (WC–17%Co) and hard chromium coatings corrosion resistances have been compared through electrochemical polarization test in 3.5% NaCl solution. WC–17%Co alloy coatings were deposited on mild steel substrates by High Velocity Oxy-Fuel (HVOF) spray process. The layers of standard and crack free hard chromium coatings were prepared by using Direct Current (DC) and Pulse Current (PC) electroplating process on the mild steel substrates. Hard chromium coatings was characterized as a reference material, to verify whether HVOF-sprayed coatings are suitable as a hard chromium coatings replacement. The microstructure of the coatings was examined by OM, SEM and XRD. Standard hard chromium coatings passivate in NaCl environment, but crack free hard chromium coatings were prepared by using Pulse Current (PC) electroplating do not passivate. The lowest corrosion current densities (Icorr) were recorded for crack free hard chromium coatings. Comparative electrochemical test results showed that, the Standard hard chromium coating has the highest Icorr and were significantly damaged after the electrochemical tests. It is seem to be that WC–17%Co alloy coatings can be substituted for standard hard chromium coatings but crack free chromium coatings bring new challenge for HVOF-sprayed coatings!

Influence of process parameters on crack formation in direct current and pulse reversal plated hard chromium

It has for a long time been known that crack free chromium coatings can be obtained by pulse reversal plating, but it has only much later been understood that reoxidation of hydrogen from the surface during the anodic periods is essential for obtaining crack free deposits. In this paper, it is shown that a specific anodic charge depending on the charge of the previous cathodic pulse is needed to obtain a crack free coating and that residual stress in the coating will be less at more frequent current reversals. Furthermore, too large an anodic charge will result in redissolution of chromium and thereby decrease the current efficiency.

Crack-free chromium as a base for electroplated gold on molybdenum pins

Crack-free chromium as a base for electroplated gold on molybdenum pins

Development and characteristics of crack-free chromium coatings produced by electroplating

SummaryCrack-free chromium coatings have been proposed for use in gun barrels to increase service life by eliminating the spalling which can occur with conventional hard chromium coatings. In this investigation variables which affect the deposition of crack-free coatings by pulse plating and DC plating with an imposed AC ripple were studied. Microstructures and hardnesses were compared with low contractile (LC) and hard chromium plate. Pulse plated coatings (ca 800–900HV) consisted of an agglomeration of small grains interspersed with pores, the size and number depending on the plating conditions. AC ripple coatings were substantially softer (ca 650HV), and had a columnar herringbone type structure at 15% ripple which changed to a granulated structure at 20%.

Friction and wear of chromium and nickel coatings

Conventional chromium plating has a considerably higher wear resistance than electroless nickel, heat-treated electroless nickel, electroplated nickel, heat-treated chromium and crack-free chromium coatings in all tests, even though it does not have the highest hardness. The ranking order of the other coatings depends upon the dominant material removal mechanism and can change from test to test. Transitions between severe and mild wear occur in electroless nickel and chromium owing to changes in phase, brittleness and scratch indentation size effects. Heat treatment effects the transition by crystallizing the amorphous electroless nickel, variation of the electroplating conditions alters the crystal structure of chromium and reduction of the scratch size changes the dominant material removal mechanism from fracture to plastic deformation. The relative merits of each coating and the role of laboratory data in coatings selection and design are discussed.

Electrochemical characterization of electrodeposited chromium and nickel-chromium layers on a steel substrate

Chromium was deposited on steel specimens and on electroplated steel specimens covered with a Watts nickel layer. The influence of the substrate and of the crack pattern of the electrodeposited chromium layer on the passivation and the reactivation behaviour has been studied by means of potentiodynamic polarization curve measurements and cyclic voltammetry. In the results of the above mentioned electrochemical measurements a strong influence of the substrate was found for a cracked chromium layer deposited on the steel specimen and for a cracked chromium layer deposited on a 30 μm Watts nickel layer. No influence of the substrate was found in the case of a crack-free chromium layer deposited on the steel substrate or on a Watts nickel layer. It is thus possible to determine whether the chromium layer is cracked or crack free by recording a polarization curve or a cyclic voltammogram for a chromium-plated system. A mechanism is proposed for the passivation and repassivation of cracked chromium layers electrodeposited on a steel substrate or on a Watts nickel layer.

Sputter deposition of 2000 Å thick crack-free chromium alloy films onto ABS plastics

Sputter deposition of 2000 Å thick crack-free chromium alloy films onto ABS plastics

Sputter deposition of 2000 Å thick crack-free chromium alloy films onto ABS plastics

Sputtering thin films of chromium and chromium alloys onto ABS plastics is a technique for producing bright work in exterior automotive applications. Because the throwing power of the sputtering source produces a four to one variation in thickness between the horizontal and vertical surfaces, a horizontal surface of at least 1200 Å may be required for acceptable metallic appearance. Films of this thickness are prone to microcracking. This paper discusses a technique to produce microcrack-free chromium alloy films by using substrate heating just prior to metallization.

The Substitution of Cobalt for Nickel in Bright Nickel Plating Solutions

SummaryThe paper describes an investigation of the full and partial substitution of cobalt for nickel in decorative nickel chromium plating systems. As the result of laboratory experiments on the effect of cobalt additions to a proprietary high chloride base bright nickel solution, two automatic plating plants were converted to cobalt-nickel alloy deposition by the introduction of cobalt anode material. One plant was operated with microcracked chromium and the other with crackfree chromium. Experience with the production plant is described. Accelerated (CASS) and static exposure corrosion tests carried out to compare the corrosion resistance of cobalt, cobalt-nickel and nickel deposits under crackfree, microcracked and microporous chromium on steel and zinc substrates are reported, together with mobile tests to compare the performance of cobalt-nickel with nickel under crackfree chromium on zinc alloy diecast headlamp rims. It is shown that the corrosion resistance of some plating systems containing cobalt is at least as good as the equivalent nickel system. Other systems suffer from staining by cobalt corrosion products. The corrosion resistance of copper/cobalt/crackfree chromium was found to be significantly better than that of copper/nickel/crackfree chromium on steel substrates.

Some Evaluations of Copper+Nickel+Chromium Decorative Systems

SummaryEvaluations and statistical comparisons have been made for various coating systems given mobile exposures in Birmingham and Cleveland. It is shown that static exposure and accelerated test results do not in all cases correspond with those obtained by mobile exposure. Systems evaluated were single and multiple layer nickel: copper-t-nickel: standard, crack-free and microporous chromium. Crack-free chromium was beneficial in Birmingham but not in Cleveland. Duplex nickel was more beneficial in Cleveland than in Birmingham. Detail is given on methods of evaluating corrosion results. The relation between sulphur content, potential and corrosion of nickel deposits is discussed. While it is not known if only a single mechanism is involved, both the open circuit potential and corrosion rate of single deposits appear to be functions of the logarithm of sulphur content.

Some Evaluations of Copper+Nickel+Chromium Decorative Systems

SummaryEvaluations and statistical comparisons have been made for various coating systems given mobile exposures in Birmingham and Cleveland. It is shown that static exposure and accelerated test results do not in all cases correspond with those obtained by mobile exposure. Systems evaluated were single and multiple layer nickel; copper+nickel; standard, crack-free and microporous chromium. Crack-free chromium was beneficial in Birmingham but not in Cleveland. Duplex nickel was more beneficial in Cleveland than in Birmingham. Detail is given on methods of evaluating corrosion results. The relation between sulphur content, potential and corrosion of nickel deposits is discussed. While it is not known if only a single mechanism is involved, both the open circuit potential and corrosion rate of single deposits appear to be functions of the logarithm of sulphur content.

Advances in Decorative Chromium Plating for Corrosion Protection of Zinc Die Castings

SummaryCrack-free chromium (0·025 or 0·05 mil thick) deposited at relatively high temperature provided excellent protection at static sites, but was only slightly better than ordinary chromium (0·01 mil) in mobile tests. Panels plated with duplex nickel (0·8 or 1·2 mil) plus ordinary or crack-free chromium suffered considerable basis-metal corrosion within two years on truck-trailers in the Michigan-Ohio area. Microcracked chromium with 1400 to 1800 cracks/in prevented basis-metal corrosion for 3½ years at all sites. Corrosion was faster on New York Harbour tugboats, but microcracked remained superior to crack-free chromium. Its superiority is attributed in part to its ability to resist damage by mechanical force.

Investigations into Corrosion Resistance of Decorative Nickel and Chromium Electrodeposits

SummaryThe results of accelerated corrosion tests, outdoor exposure tests and more fundamental electro-chemical studies have been critically compared as a means of predicting corrosion resistance of nickel+chromium electrodeposits in service. The aim was to compare the relative merits in British atmospheres of some of the various alternative nickel and chromium systems available and to ascertain which of the commonly used accelerated tests placed these systems in the same order of merit as did outdoor exposure. All tests were performed with plated steel panels, some of which were deformed before exposure. The coating systems which gave the best corrosion protection in all outdoor trials were found always to include the thicker chromium deposits, but while crack-free chromium was best on static exposure, micro-cracked chromium was good in both static and mobile conditions. Duplex nickel gave much better protection during mobile tests than on static exposure. The Corrodkote test was the most generally applicable rapid accelerated corrosion test for assessing plated steel. The acetic acid salt spray test gave results which correlated most closely with those obtained on outdoor exposure but takes longer to perform.

The Effect of Deformation on the Corrosion Resistance of Nickel + Chromium Coatings: A Metallographic Study

SummaryA study has been made of the incidence of cracking in deformed nickel + chromium plated panels and of the relation to cracking corrosion behaviour. After very severe deformation, only semi-bright nickel remained completely free from cracking. Bright nickel cracked readily, and in heavily deformed regions the cracks tended to propagate, by notch effect, through ductile semi-bright nickel or copper undercoats. In specimens less severely deformed, cracking terminated at the semi-bright nickel/bright nickel or the copper/bright nickel interface. Thick, initially crack-free chromium coatings cracked over a wider area and induced many more cracks in bright nickel substrates than did chromium of conventional thickness, whereas over similar substrates thick microcracked chromium coatings induced fewer cracks than conventional chromium. In all test environments, wide cracks which terminated at an active substrate such as steel or zinc usually showed corrosion, of the substrate only, but in stationary and mobile exposure tests corrosion of both the bright nickel and the active substrate tended to occur in narrow cracks. In acetic-acid/salt spray, however, corrosion in even the narrowest cracks was confined to the active substrate. With increasing nobility of substrate, in the order brass, copper, semi-bright nickel, the proportion of corrosion occurring in the substrate decreased and attack was confined to the bright nickel layer.

Corrosion Resistance of Chromium-Plated Zinc Alloy Die Castings

SummaryThe benefits of thicker, crack-free chromium have been confirmed in several exposure tests. Production trials have demonstrated that the high-temperature high-ratio process involves no major technical difficulties and no large increase in cost. No significant improvement in performance was observed using duplex instead of single-layer bright nickel; sacrificial corrosion of the upper layer of the duplex coating occurred, but delayed penetration only briefly and gave rise to surface pits which were themselves unsightly. A duplex coating might show to greater advantage on steel articles, both because its superior ductility enables it to resist distortion and because the postponement of rusting is perhaps more important than a delay in the first appearance of zinc corrosion product. Dual, microcracked chromium showed variable corrosion resistance. At its best, it greatly delayed the occurrence of basis metal corrosion; it suffered a slow permanent loss of lustre, however, and was liable to stain in humid atmospheres. Sandwich chromium coatings were also variable in performance, and most samples showed at least a few sites of basis metal attack. The shallow pits formed in the upper layers of these coatings were unobtrusive so long as they were not too numerous. No doubt, a more consistent performance could be achieved, but the cost of this process must remain high.