Chromate Pretreatment Research Articles

Corrosion of coated aluminium alloy 7075-T6 galvanic test assemblies, part I: unscribed panels

ABSTRACTGalvanic corrosion of samples consisting of coated aluminium alloy 7075-T6 panels connected to stainless steel 316 fasteners was examined. Two coating systems were evaluated, one containing chromate pretreatment and primer along with a topcoat, and the other a chromate free system. No intentional defects such as scribes were imparted to investigate the attack associated with intrinsic defects in the coatings. The galvanic current between the fasteners and panel was greater for the assembly with chromate-free coatings than for the one with chromated coatings, and for the exposure to a salt spray chamber than for immersion. The attack was much deeper for the chromated system and more localised. The galvanic current increased with time and then was equal to that measured for a sample with an intentional scribe.

Microstructural and Compositional Characterisation of Chromate Pretreatment on Aluminium

Chromate conversion coating developed on aluminium has been examined using SEM/EDX and CTEM/EDX with a view to having further knowledge of its intrinsic surface, plan, and sectional morphologies which will aid the understanding of their roles in improved corrosion and adhesion properties of the underlying substrate. The surface consists of spherical clusters of particulate materials. The sections, however, reveal approximately parallel, linear features which terminate at or close to the metal/coating interface, while plan views show cell-like particulate features. The coating is composed of chromium and aluminium compounds, both, probably hydrated. For a conversion coating to fully replace its chromate counterpart, most of these features may have to be replicated in the nonchromium coating material which should contain nontoxic, leachable corrosion inhibiting species.

The use of sol–gel film as pre-treatment for tinplate used in the canning industry

Tinplate cans are extensively used in the food packaging industry. The regular practice includes a passivation process, usually based on chromate films. Although they have good performance, their contaminant and unhealthy character makes necessary the research of new alternatives.The present work proposes the use of films based on vinyltrimethoxysilane precursor, synthesized by sol–gel technique, as an alternative to the traditional chromate pre-treatments. Different techniques were employed to characterize the obtained film: Fourier-transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray, mechanical profilometry and X-ray diffraction. The corrosion resistance was evaluated by polarization tests and electrochemical impedance spectroscopy.The obtained results suggest that the new technology can offer higher corrosion resistance at short immersion time; nevertheless, it is necessary to improve the formulation in order to obtain better performance.

Corrosion resistance of organic coating pretreated by phytic acid

In this study, phytic acid pretreatment is used as a substitute for toxic chromate pretreatment to improve the corrosion resistance of organic coating on AZ91D magnesium alloys. The microstructures of phytic acid conversion films are observed by a transmission electron microscope. The role of phytic acid pretreatment on the corrosion resistance of organic coating is investigated using salt spray test and electrochemical impedance spectroscopy. The experimental results show that the phytic acid conversion film exhibits a dense amorphous structure and phytic acid pretreatment can effectively improve the corrosion resistance of organic coating on magnesium alloys.

Method to improve corrosion resistance of AA 5083 by cerium based conversion coating and anodic polarisation in molybdate solution

The corrosion resistance of aluminium alloys can be improved by the formation of chemical conversion coatings. Usually, chromate pretreatment processes are used. However, because of their high toxicity, alternative methods have been studied in the last years. In the present paper, a process to improve the corrosion resistance of AA 5083 was investigated. The main process consisted of serial immersion in two solutions containing two different cerium salts, followed by a further step involving anodisation in Na2MoO4. The corrosion resistance was investigated by potentiodynamic anodic polarisation tests and electrochemical impedance spectroscopy. The morphology and composition of the oxide layers were characterised by scanning electronic microscopy equipped with an energy dispersive X-ray detector. Serial treatment in the two solutions with different cerium salts and H2O2 produced a more uniform and thicker layer with a larger amount of deposits above the alloy cathodic areas than the single two treatments. In this way, the alloy corrosion resistance in chloride environments was improved and decreased the anodic reaction rate. When anodisation in molybdate salts was added, even better results were found. The corrosion resistance was higher due to a thicker and less conductive passive layer.

Fusion bonded epoxy mainline and field joint coatings performance from the X100 field trial – A case study

Operating and distribution companies are potentially interested in the use of high and ultra-high strength steels for the transportation of high pressure gas. The ultra-high strength X100 grade steel was commercially developed as a potential option to meet this. However, there has been limited industry wide use of X100 to date.BP carried out a 2 year field trial to demonstrate the operational capacity and integrity of a large diameter (48 inch/1219 mm) high pressure pipeline constructed from X100 grade steel. The 800 m pipeline was buried in a clay backfill and exposed to wet ground conditions associated with the North of England. Flow pressure cycling was carried out, using water, to simulate 40 years of operational service.A 200 m section of the pipeline was exposed to three different potential (cathodic protection) zones for the duration of the trial: zero potential, intermediate potential (−850 to −950 mV) and high potential (−1200 to −1300 mV). This section also had damage and defects induced which are typically associated with bad installation and commissioning.An area of potential concern is the degradation of the mechanical properties (strain ageing) due to the external coating application temperature. Thus, a low coating application temperature is deemed desirable.The mainline and field joint coatings employed for the trial were fusion bonded epoxy (FBE). Both of these have been used in other BP projects, with a good track record. They were applied at a lower application temperature of 220 °C, compared to the more typical 230–240 °C. The lower application temperature was within the manufacturers approved application and curing temperature range. The lower temperature was used to assess the ultimate performance properties of the mainline and field joint FBE coatings.Mainline and field joint coating samples were taken from the three different potential zones and extensive testing and characterisation carried out. This paper presents and discusses the results of the testing.It was found that the different potential zones had negligible effect on the properties of the mainline and field joint coatings. The post-field trial properties of the mainline coatings were unaltered (i.e. same as pre-field trial) despite burial for over 2 years.Although the mechanical properties (flexibility and impact resistance) of the field joint coatings were acceptable, the coatings' water soak and cathodic disbondment resistance were far inferior to those of the mainline coating. This can be related to the comparably lower adhesion, and directly attributed to the adhesion promoting chromate pre-treatment used for mainline coatings, but not so for field joints. This is stipulated by environmental legislation. This is an area which needs to be addressed prior to X100 being used in any future project.

Improvement of corrosion protection offered to galvanized steel by incorporation of lanthanide modified nanoclays in silane layer

Silane sol–gel based films are very promising alternatives to the traditional chromate pre-treatments. However, the protection offered by the silane films strongly decreases when the coating is damaged. Some previous studies showed that the barrier properties of the silane layer can be improved by incorporation of clay nanoparticles. Moreover, inhibitive metallic cations can be incorporated in the nanoclays by ion exchange, providing a way to prepare cheap corrosion inhibitors nanoreservoirs offering self-healing properties. Rare earth (RE) metal salts have been shown to be effective corrosion inhibitors on a wide range of metals, including hot dip galvanized (HDG) steel. For this study, montmorillonite clay is modified to obtain a Ce(III) montmorillonite clay (Ce-MMT). The amount of incorporated Ce(III) is characterized by means of XRF measurements. X-ray diffraction showed that the Ce(III) is located in the interlayer space.The influence of the ionic strength and of the nature of the counter ion on the kinetic of Ce(III) release is investigated in NaCl and ZnCl2 solutions.Either unmodified MMT or Ce-MMT nanoreservoirs are incorporated in a silane sol–gel treatment consisting in a combination of 3 different silanes (GPS, TEOS and MTES).The corrosion protection offered to HDG substrate by the modified silane layer was evaluated by means of electrochemical impedance spectroscopy in 0.1M NaCl.

Effect of Pretreatment Film Composition on Adhesion of Organic Film on Zinc Coated Steel Sheet

It is known that chromate pretreatment films with dry-type silica adhere well to organic films such as paint and laminate films on a zinc coated steel sheet. The role of surface polarity in the adhesion mechanism has been investigated for a long time, but remains unclear. We examined the correlation between the composition of chromate pretreatment films with dry-type silica and their adhesion to a laminate film on a zinc coated steel sheet, for both the wet silica and without silica types of chromate pretreatment film. We also investigated the adhesion mechanism by analyzing the pretreatment films using SEM, AFM, BET, TEM and EDX. The results were as follows. Not only the type and particle size of silica, but also the silica/chromate ratio and the coexistence of chromate affected the adhesion of laminate films. The highest adhesive strength was obtained for the pretreatment films formed in a pretreatment solution with SiO 2 /Cr ratio of 4.0 and dry-type silica with a particle diameter of 7 nm. The good adhesion of chromate pretreatment films with dry-type silica is due to an anchor effect by sub-micron surface roughness and an increased number of interface bonding sites by a nano-porous structure. The formation of an insoluble Cr(III) compound layer plays an important role in binding silica particles to the zinc coating surface.

亜鉛めっき鋼板上の有機皮膜との密着性に及ぼす前処理皮膜組成の影響

It is known that chromate pretreatment films with dry type silica have a good adhesion to organic films such as paint and laminate films on a zinc coated steel sheet. The adhesive mechanism has been investigated from the standpoint of surface polarity for a long time, but has not been perfectly explicable. In this work, a correlation between the composition of chromate pretreatment films with dry type silica and the adhesion to a laminate film on a zinc coated steel sheet was examined in comparison with the chromate pretreatment films with wet type silica and without silica. Furthermore the adhesive mechanism has been investigated by the analysis of the pretreatment films using SEM, AFM, BET method, TEM and EDX. The results are summarized as follows: Not only the type and the particle size of silica, but also the silica/chromate ratio and the coexistence of chromate affected the adhesion of laminate films. The highest adhesive strength was obtained for the pretreatment films formed in a pretreatment solution with SiO2/Cr ratio of 4.0 and dry type silica with a diameter of 7 nm. The good adhesion of chromate pretreatment films with dry type silica is due to an anchor effect by sub-micron surface roughness and an increased in number of interface bonding sites by a nano-porous structure. The formation of an insoluble Cr(III) compound layer plays an important role in binding silica particles to the zinc coating surface.

Oxidation Control with Chromate Pretreatment of MCrAlY Unmelted Particle and Bond Coat in Thermal Barrier Systems

MCrAlY alloy bond coat is widely used in thermal barrier coating (TBC) systems to protect substrates from high-temperature oxidizing environments. However, failure of the ceramic topcoat can occur due to a thermally grown oxide (TGO) that grows at the interface between the bond coat and the topcoat. In this study, the effect of chromate treatment was investigated. Prior to topcoat deposition, a thin film of Cr2O3 was formed on the bond coat surface. High-temperature oxidation tests were carried out, and the oxidation rates were determined by inspection of cross sections. Similar oxidation tests were carried out using MCrAlY powder material assumed to be unmelted particles. As a result, the chromate-treated bond coat showed outstanding oxidation resistance. Calculations that take into account the oxidation of particles in the topcoat indicated the generation of internal stress to cause local fracture of the topcoat.

Failure characteristics of adhesively bonded aluminium for spacecraft applications

AbstractThe surface characteristics of an aluminium alloy treated with a commercial chromate pretreatment have been studied by SAM, EDX, and XPS. XPS indicates that chromium is present in the conversion coating as both Cr(III) and Cr(VI) species, and that other components are present at the conversion‐coating surface, some in a reduced form. The deposition of the chromate layer is compromised of the presence of the intermetallics in the alloy, and the layer is cracked at the micrometer scale, presumably as a result of drying on removal from the bath. Failure of chromate‐treated aluminium bonded in lap‐shear configuration using a structural epoxy paste is shown to be cohesive in the adhesive but very close to the conversion coating/adhesive interface but alternating from one side of the glue line to the other. Copyright © 2008 John Wiley & Sons, Ltd.

Development of nanostructured polyaniline dispersed smart anticorrosive composite coatings

AbstractModern engineering science and nanotechnology have hastened the development of high performance corrosion‐resistant coatings having a broad spectrum of effectivity under a wider range of hostile environments. The formulation of such coating systems is expected to cause a major revolution in the corrosion world. Conducting polymers have recently proved to be an effective alternative to phosphate–chromate pretreatment that is hazardous due to toxic hexavalent chromium. Moreover, improvements in environmental impact can be achieved by utilizing nanostructured particulates in coating and eliminating the requirement of toxic solvents. The paper reports some preliminary investigations on the corrosion resistance performance of nanostructured methyl orange (MO)‐doped polyaniline (PANI)/castor oil polyurethane (COPU) composite coatings on mild steel (MS). The nanostructure of the MO‐PANI was confirmed by transmission electron microscopy (TEM). The corrosion protective performance was evaluated by physico‐mechanical properties, corrosion rate, and open circuit potential measurements. These coatings were found to act as “corrosion sensors” by exhibiting different colors when placed in acid as well as alkaline media. The protective behavior of coatings was attributed to the formation of a passive iron oxide/dopant layer at the metal‐coating interface that impedes the penetration of the corrosive ions. Copyright © 2008 John Wiley & Sons, Ltd.

Nanostructured sol–gel coatings doped with cerium nitrate as pre-treatments for AA2024-T3: Corrosion protection performance

Nanostructured hybrid sol–gel coatings doped with cerium ions were investigated in the present work as pre-treatments for the AA2024-T3 alloy. The sol–gel films have been synthesized from tetraethylorthosilicate (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) precursors. Additionally the hybrid sol was doped with zirconia nanoparticles prepared from hydrolyzed tetra- n-propoxyzirconium (TPOZ). Cerium nitrate, as corrosion inhibitor, was added into the hybrid matrix or into the oxide nanoparticles. The chemical composition and the structure of the hybrid sol–gel films were studied by XPS (X-ray photoelectron spectroscopy) and AFM (atomic force microscopy), respectively. The evolution of the corrosion protection properties of the sol–gel films was studied by EIS (electrochemical impedance spectroscopy), which can provide quantitative information on the role of the different pre-treatments. Different equivalent circuits, for different stages of the corrosion processes, were used in order to model the coating degradation. The models were supported by SEM (scanning electron microscopy) measurements. The results show that the sol–gel films containing zirconia nanoparticles present improved barrier properties. Doping the hybrid nanostructured sol–gel coatings with cerium nitrate leads to additional improvement of the corrosion protection. The zirconia particles present in the sol–gel matrix seem to act as nanoreservoirs providing a prolonged release of cerium ions. The nanostructured sol–gel films doped with cerium nitrate can be proposed as a potential candidate for substitution of the chromate pre-treatments for AA2024-T3.

Formation and characterisation of a chromate conversion coating on AA6060 aluminium

AA6060-T6, an AlMg0.5Si0.4 model alloy and α-Al(Fe,Mn)Si phase electrodes have been subjected to chromate treatment in a commercial chromate-fluoride based solution. The coated surfaces were subsequently examined by use of field emission SEM, TEM, AES and electrochemical measurements in 0.1 M NaCl solution in order to study the effect of substrate microstructure on coating formation and properties. Non-uniform growth of the chromate conversion coating (CCC) on AA6060-T6 resulted in a porous morphology, with cracks extending down to the base metal. Poor coverage was particularly observed at the grain boundaries. The thickness of the CCC after completed treatment was about 150–200 nm, while significantly thinner films were formed on the α-Al(Fe,Mn)Si particles. AlMg0.5Si0.4 in the artificially aged (T6) condition exhibited a coating morphology similar to AA6060-T6, while CCC formation on homogenised AlMg0.5Si0.4 was characterised by growth of localised oxide particles and filaments, resulting in poor coverage. These observations indicated that precipitation of Mg 2Si particles due to heat treatment promoted nucleation of the CCC. Chromate treatment caused a significant reduction of cathodic activity on AA6060 during subsequent polarisation in chloride solution. This was attributed to rapid formation of a thin chromium oxide film on the α-Al(Fe,Mn)Si particles during the chromate treatment, resulting in significant cathodic passivation of the phase. Inhibition of the oxygen reduction reaction at cathodic intermetallic particles is suggested as an important factor contributing to the high performance of chromate pre-treatments on aluminium.

MCrAlY (M: Ni, Co) ボンドコート上の未溶融粒子の酸化現象とトップコートの力学的挙動に対する影響

A MCrAlY alloy bond coat is widely used in thermal barrier coating (TBC) systems to protect substrates from high-temperature oxidizing environments. However, failure of the ceramic top coat can occur due to a thermally grown oxide (TGO) layer that grows at the interface between the bond coat and the top coat. Local stresses are produced by multiple oxides that grow separately at the interface. It seems that the multiple oxides may originate in the rapid oxidation of unmelted MCrAlY particles attached to the bond coat during spraying. Thus, it has become an important issue to control the growth behavior of TGOs.In the present study, the effect of chromate treatment was investigated. Prior to top coat deposition, a thin film of Cr2O3 was formed on the bond coat surface. High-temperature oxidation tests were carried out using samples coated both with and without chromate pretreatment, and the oxidation rates were determined by inspection of cross sections. Similar oxidation tests were carried out using MCrAlY powder material assumed to be unmelted particles. Chromate-treated bond coat showed outstanding oxidation resistance in comparison with bond coat without chromate pretreatment. In the case of pretreated MCrAlY powder, only the surfaces of particles were oxidized, even under conditions in which most particles without pretreatment were fully oxidized. This outcome is thought to result from the preferential formation of an oxidation-resistant Al2O3 layer in the early stage of oxidation. Calculations that take into account the oxidation of particles in the top coat indicate the generation of sufficient internal tensile stress to cause local fracture of the top coat. Because of its simplicity and applicability to complicated shapes, the chromate treatment is quite useful as a countermeasure against bond-coat oxidation and local fracture of the top coat near the interface.

Polyurethane/Polysiloxane Ceramer Coatings: Evaluation of Corrosion Protection

A series of polyurea and polyurethane ceramer coatings were formulated using hexamethylene diisocyanate (HDI) isocyanurate, alkoxysilane-functionalized HDI isocyanurate, tetraethyl orthosilicate (TEOS) oligomers and cycloaliphatic polyesters. The coatings were prepared as a function of alkoxysilane-functionalized HDI isocyanurate and TEOS oligomers concentration. Also, the effect of acid catalyst was investigated. The corrosion resistance of polyurea or polyurethane ceramer coating systems were evaluated using a prohesion chamber on aluminium alloy 2024-T3 substrate. The polyurethane ceramer coatings were compared with the chromate pretreatment and the epoxy-polyamide primer containing the chromate pigment. In addition to prohesion, the interface between the coating and substrate was characterized using X-ray photoelectron spectroscopy (XPS). The prohesion data showed that the corrosion was inhibited by the TEOS oligomers. However, high concentrations of TEOS oligomers and acid catalyst produced blistering in the polyurea/polysiloxane ceramer coatings. The prohesion data also showed that the corrosion protection of ceramer coatings performed as well as the chromate pretreatment and competitively with the epoxy primer. From the XPS and prohesion data, a self assembling silicon oxide layer at the metal-coating interface was proposed.

Properties of ketimine/acetoacetate coated aluminum substrates

Based on ketimine/acetoacetate coating technology, a new two-component high solids primer for aerospace use has been developed. The objective was to formulate a chromate-free primer that may be applied to all relevant aluminum surfaces, particularly after chromate-free pretreatment. During development, interesting indications of good adhesion and good corrosion resistance were found. For confirmation and in order to optimize the primer, research was focused on the interfacial performance of the coatings on typical aerospace substrates. In this paper, therefore, the nature of the substrates, including their pretreatments, is discussed with initial emphasis on the nitric acid pickling pretreatment, used in place of chromate pretreatments with their desirable properties. Although aging in relevant electrolytes of such pretreated aluminum leads to an increase of the thin oxide layer by further hydration normally influencing negatively interfacial performances, cross cut adhesion values of the primer, or the primer/topcoat, before and after aging in several electrolytes, were good on clad as well as on non-clad material. This was confirmed by microscopic morphology assessment during similar aging processes of the coating/substrate systems. The hydration of the nitric acid pickled layer was not found in the presence of primer, with or without topcoat. Using electron energy loss spectroscopy, indications of a distinct chemical bond between the primer and the nitric acid pickled aluminum were revealed. Electrochemical impedance measurements, determining film resistances of the various coated panels, also showed good responses when the primer was pigmented with a selected chromate-free inhibitor. Finally, in line with the previous indications and due to the special interfacial performance, in the presence of chromate-free pretreated substrates, filiform corrosion resistance with chromate alternative pigmentation approaches that of chromated primers.

EIS study of environmentally friendly coil coating performances

The corrosion protection of metal structures by the application of organic coatings is obtained using complex paint cycles. The metal pretreatment as well as primer, intermediate and top coat are the usual layers composing the protection system. Although chromate pretreatment processes and chromate containing primers are widely used, they require highly toxic chromic acid solutions, with consequent effluent disposal and ecological problems. This fact pushed many research laboratories to develop new chromium-free pretreatments and primer inhibitors. Electrochemical impedance spectroscopy (EIS) proved to be a very useful technique to study the protection properties of the organic coating because of the possibility of evaluating the corrosion process on the metal substrate and to measure the electric and dielectric characteristics of the coating. In this study, new chromium-free pretreatments and new primers containing ecological inhibitors were applied to steel substrates coated using an Al-Zn alloy. The results were compared with those obtained using traditional protection systems. The characterization of the complete system (pretreatment and primer) in sodium sulfate solution clearly showed the different water uptake behavior of the two primers, which highlights the better barrier properties of the one containing phosphate. However the corrosion protection of the primer containing chromates is also due to the inhibitive action of the chromates. Hence such a type of primer better withstands the presence of defects in the coating. Moreover the presence of environmentally friendly pretreatments do not worsen the corrosion protection which appears comparable to that observed when chromatation is employed.

Surface study of a chromate pretreatment before coil coating of Al–43.4Zn–1.6Si coated steel

AbstractChromating is frequently used for temporary corrosion protection and as a pretreatment before organic coating of aluminium, zinc and aluminium/zinc alloys. In this work we have studied the composition and the chemical state of pretreated Al–43.4Zn–1.6Su‐coated strip steel. Auger and x‐ray photoelectron spectroscopy were used to characterize the surface of samples exposed to the chemicals from the different processing steps. In the first step, which is alkaline cleaning, hydroxy ions in the cleaner dissolve aluminium oxide from the surface of the strip and make the surface richer in zinc. In the chromating step that follows, the chromic acid causes dissolution of Al and Zn from the surface and precipitation of a complex chromium hydroxide layer containing small amounts of fluoride, aluminium, tungsten and molybdenum ions. Chromium exists in the valance states Cr3+ and Cr6+. In the final pretreatment step, which consists of a chromic acid after‐rinse, additional, mainly hexavalent, chromium is deposited. The chromate coating is thicker in regions indented by the temper rolling than in surrounding areas. The temper rolling, which is performed with textured rolls after metallization of the strip, breaks the surface aluminium oxide film and increases the reactivity of indented regions. The chromate coating is also thicker in the Zn‐rich interdendritic regions than on the Al‐rich dendrite arms. This is probably due to the microstructure of the metal coating in interdendritic areas.

塗装前処理としての塗布型クロメート

塗装前処理としての塗布型クロメート