Investigation of the chromate conversion coating on Alclad 2024 aluminium alloy: effect of the pH of the chromate bath

The parameters of the chromate bath, like temperature, pH, and fluoride content, strongly affect the morphology and chemical composition of the chromate conversion coating and as a consequence have a large influence on its corrosion performance. In this paper, electrochemical impedance spectroscopy (EIS) was used in combination with other techniques to investigate the role played by the pH of the chromate bath on the properties of the chromate film formed on Alclad 2024 aluminium alloy. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE) have shown the formation of a thicker and less dense chromate layer when the pH of the chromate bath is changed from 2.4 to 1.2. The analysis of the EIS spectra have highlighted that this change in pH leads to the formation of more protective and more resistant chromate corrosion products (CCP) inside the defects of the chromate film. When a thin, dense and protective layer of CCP is formed in the defects, the corrosion behaviour of the chromate conversion coating improves for two main reason: (a) further attack of the defects is avoided or delayed; (b) the change in volume caused by the formation of the CCP is limited resulting in a low level of stress in the film, which as a consequence is not detached from the aluminium substrate.     

Testing and evaluation of nonchromated coating systems for aerospace applications

The advanced corrosion resistant aircraft coatings program (ACRAC) is part of the Air Force strategy to improve performance and reduce environmental impact of coatings used on Air Force weapon systems. The program addresses the Air Force near and mid-term strategies to eliminate chromate corrosion inhibitors and reduce steps in the outer mold line coating process. Evaluation of the coating process (surface preparation, conversion coating, primer, topcoat) as a system is a key feature of the ACRAC program. Results to date indicate that the current-state-of-the-art nonchromated coating systems are significantly less effective than those with chromate. A chromate conversion coating is required for the nonchromate primer system to meet minimum requirements. Sol–gel-process based conversion coatings can replace chromate conversion coatings provided a chromated primer is used. Several approaches to incorporating inhibitors into sol–gel coatings are discussed. Electrochemical methods for testing coating performance are discussed and a new procedure based on impedance spectroscopy for evaluating active damage repair is presented.     

An organically modified zirconate film as a corrosion-resistant treatment for aluminum 2024-T3

Current coating systems for aircraft corrosion protection are based on a traditional chromate surface treatment, primer, and topcoat. The Air Force is seeking environmentally benign corrosion-resistant surface treatments for aluminum-skinned aircraft as a replacement for environmentally hazardous surface treatments involving chromates. Performance of replacement treatments must be able to satisfy the durability needed for dramatically extended lifetimes, be compatible with present and future environmental requirements, and be easily integrated into current primer/topcoat paint systems.

Organically modified zirconate sol–gel films were investigated as an environmentally compliant replacement for chromated surface treatments, which included functionalized components to tailor the chemistry at both the aluminum oxide substrate/surface treatment interface and the surface treatment/topcoat interface. Sol–gel films were applied to aluminum 2024-T3 alloy coupons, and the resulting film chemistry was investigated. FTIR was used to identify organic components in the film and X-ray photoelectron spectroscopy was used to investigate the interface chemistry. The result of the chemically modified sol–gel synthesis was a coating in which a concentration gradient was formed at the surface, which is discussed. The corrosion protection attributes of these films was also studied and the electrochemical evaluation of sol–gel films will be discussed, both as stand-alone coatings and as part of a full coating system.

Organically modified sol–gels exhibited significantly better protection in terms of barrier properties in comparison to a typical chromate-based processes. The resulting data from evaluations of sol–gel produced coatings show promise towards the goal of producing a robust chemical interaction/bonding of such corrosion-resistant coatings on the surface of aluminum-skinned aircraft without the use of environmentally hazardous chromate agents.     

The role of chromate in filiform corrosion inhibition

This paper is part of a study on filiform corrosion (FFC) on aluminum alloy 2024-T3 and focuses on the surface characterization of corroded samples. Untreated samples were used as well as samples which had undergone pretreatments including polishing, surface etching and chromated conversion coatings (CCC). These samples were coated with both pigmented and non-pigmented epoxy-based coatings. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis was used to investigate the nature of the surface as well as optical images to gauge the rate of FFC advancement. FFC corrosion rates decreased on samples which had received a surface etching pretreatment and a chromate conversion coating. Pigmented coatings reduced the rate of FFC further and led to two different types of corrosion surface morphology. On pretreated samples, the corrosion appeared deeper and pit-like, possibly due to an enhanced polymer-substrate bond. On untreated samples, widespread FFC developed. SEM and EDX analysis of various intermetallic particles (IMPs) on all samples, inside and outside of corroded regions, revealed that Cr from pigments was found deposited on Cu-containing intermetallics in corroded areas while Fe/Mn-containing particles were free from all pigment traces. These results suggest that the Cr deposition on Cu particles hinders the cathodic reduction of oxygen necessary for FFC advancement. For this reason, the coating pigments proved, under FFC conditions, to be a more effective inhibitor than the Cr originating from a chromate conversion coating.     

The formation and corrosion behavior of a zirconium-based conversion coating on the aluminum alloy AA6061

To understand the mechanism of the coating formation, the formation process of a zirconium-based conversion coating on aluminum alloy 6061 has been studied by means of AFM in PeakForce Kelvin Probe Force Microscope (PF-KPFM) mode which could provide direct evidence for the existence of driving force for the film formation. In addition, various techniques including SEM, XPS, EIS, salt spray test, and scanning electrochemical microscope were used to investigate the surface state and corrosion behavior of the conversion film. The direct driving force for the coating formation is the Volta potential difference between the intermetallic particles and matrix. That difference produces an ocean of micro electrochemical cells in which the intermetallic particles act as cathodic sites for the film deposition. However, the precipitation of the layer is a self-limited process in which the driving force gradually decreases as the conversion layer covers the surface of the aluminum alloys. The anti-corrosion performance of the film is unfavorable compared to the conventional chromate conversion coatings due to the pitting corrosion that occurs when exposed to harsh environment containing chloride. Furthermore, the zirconium-based conversion coating possesses no self-healing ability leading to the continuous degradation of the film until it completely lose efficacy.     

Synthesis and characterisation of surface gradient thin conversion films on zinc coated steel

Surface gradient layers on hot-dip galvanised steel were synthesised in order to determine the barrier properties and corrosion resistance of thin amorphous conversion coatings as a function of layer thickness and processing time. For this purpose, a dip coating procedure was established that yields well-defined gradient layers. As a model system for conversion film formation on zinc coated steel, a zirconium based bath chemistry was used. The synthesised zirconium oxyhydroxide gradient films were characterised by localised electrochemical techniques, such as Scanning Kelvin Probe (SKP) and electrochemical impedance spectroscopy using an electrochemical capillary cell. Microscopic infrared reflection absorption spectroscopy (μ-FT-IRRAS) measurements and small-spot X-ray photo electron spectroscopy (XPS) were used as complementary surface analytical techniques. The applied analysing techniques provide a spatial resolution of 100-1600 μm. Thereby, a complete variation of thin film properties, such as thickness, barrier properties, corrosion resistance and chemical composition can be measured as function of the time of film growth on a sample with a length of a few centimetres. This approach allows a precise and accurate determination of structure-to-property relationships of thin conversion films. Moreover, it could be shown that a surface gradient film analysis significantly rationalises experimental time and increases the reliability of the experimental results.     

锌基压铸件直接低铬钝化工艺研究

由于锌基压铸件表面缺陷多,致密结晶层薄,建议在其上直接低铬彩色钝化,工艺包括;1)采用低碱性混和清洗液除油和三步法去锈,以获得干净光亮表面,同时防止过腐蚀;2)浸入合适的低铬钝化液中钝化。所得钝化膜彩色鲜艳,与镀锌钝化膜相比,具有优良的抗工业性大气腐蚀性能。     

热浸镀锌层表面偏钒酸盐转化膜

在热浸镀锌试样表面获得了一层均匀、完整的偏钒酸盐转化膜.成膜溶液成分及工艺条件为:NaVO3 5 g/L,pH 1.3,温度30 ℃,时间30 min.对比研究了偏钒酸盐转化膜和铬酸盐转化膜的耐蚀性能.结果表明,偏钒酸盐转化膜由Zn、O、V等元素组成,热浸镀锌层经偏钒酸盐转化处理后电化学阻抗和极化电阻增大,腐蚀电流密度...     

Development of an environmentally friendly protective coating for the depleted uranium-0.75 wt% titanium alloy: Part III: Surface analysis of the coating

Molybdenum oxide-based conversion coatings have been formed on the surface of the depleted uranium-0.75 wt% titanium alloy using either concentrated nitric acid or fluorides for surface activation prior to coating formation. The acid-activated surface forms a coating that offers corrosion protection after a period of aging, when uranium species have migrated to the surface. X-ray photoelectron spectroscopy (XPS) revealed that the protective coating is primarily a polymolybdate bound to a uranyl ion. Rutherford backscattering spectroscopy (RBS) on the acid-activated coatings also shows uranium dioxide migrating to the surface. The fluoride-activated surface does not form a protective coating and there are no uranium species on the surface as indicated by XPS. The coating on the fluoride-activated samples has been found to contain a mixture of molybdenum oxides of which the main component is molybdenum trioxide and a minor component of an Mo(V) oxide.     

Corrosion behaviour of electrogalvanized steel in sodium chloride and ammonium sulphate solutions; a study by E.I.S.

Zinc and zinc-nickel (13% Ni) electrodeposits were passivated by dipping in chromate baths and characterized by scanning electron microscopy. The corrosion behaviour was studied using a.c. electrochemical techniques; electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit and under galvanostatic control during the 24 h immersion time. In sodium chloride solution the zinc-nickel electrodeposits show a better corrosion resistance compared to the pure zinc coatings. During the immersion time, a surface nickel enrichment was observed which, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate. In the same solution the passivation treatment improves the corrosion resistance of the electrodeposits; nevertheless, on zinc substrates, the protection exerted by the chromate film is not, always effective during the immersion time. On the contrary the chromate coating on zinc-nickel substrates induces a remarkable and durable improvement of the corrosion resistance reducing the zinc dissolution almost completely. In the ammonium sulphate solution, the corrosion mechanism is significantly influenced by hydrogen reduction on the zinc-nickel surfaces, and by the production of a local surface acidity which is aggressive for the chromate coatings.     

Novel Method for Zirconium Oxide Synthesis from Aqueous Solution

A novel method for zirconium oxide crystal synthesis from an aqueous solution at ordinary temperature and pressure has been discovered. Boric acid was added to a sodium hexafluorozirconate solution, fluoride ion was consumed by the formation of BF⁻₄ complex ion, then the hexafluorozirconate ion was hydrolyzed to zirconium oxide in order to increase the amount of fluoride ion. The formation of zirconium oxide crystal was confirmed by clear X-ray diffraction peaks. This method is promising for preparing zirconium oxide films in various applications     

Fabrication of hydrophobic surface with hierarchical structure on Mg alloy and its corrosion resistance

A hydrotalcite/hydromagnesite conversion coating with hierarchical structure has been fabricated on a Mg alloy substrate by in situ hydrothermal crystallization method. A MgO layer existing between the hydrotalcite/hydromagnesite film and the substrate was formed prior to the hydrotalcite/hydromagnesite film during the crystallization process. After surface treatment with silane coupling agent, the surface of conversion coating changes from hydrophilic to hydrophobic. Scanning electron microscopy (SEM) revealed that the silylated conversion coating with hierarchical structure maintains the original rough surface of which was composed of numerous micro-scale flakes and beautiful flower-like protrusions. Polarization measurements have shown that the hydrophobic conversion coating exhibited a low corrosion current density value of 0.432 μA/cm², which means that the hydrophobic conversion coating can effectively protect Mg alloy from corrosion. Electrochemical impedance spectroscopy (EIS) showed that the impedance of the hydrophobic conversion coating was 9000 Ω. It means that the coating served as a passive layer with high charge transfer resistance.     

Formation of ultra-thin amorphous conversion films on zinc alloy coatings: Part 2: Nucleation, growth and properties of inorganic-organic ultra-thin hybrid films

Within the two parts of this contribution a detailed investigation of the formation of ultra-thin amorphous conversion coatings on hot dip galvanised steel is reported. The second part deals with the nucleation, growth and the barrier properties of these ultra-thin films on model substrates. The chemical composition as well as the film growth kinetics of the phosphoric acid and complexing organic macromolecules within the conversion layer system on a hot dip galvanised steel substrate have been determined by infra-red reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). Film nucleation was studied by atomic force microscopy (AFM) that revealed the initial formation of individual nanoscopic islands, which depends strongly on the initial surface chemistry. Cyclovoltammetry (CV) and potentiostat measurements were performed to characterise the blocking of ion transfer and electron transfer reactions on the modified surface. An excellent correlation with the barrier properties of the film with the state of film growth and the composition of the inorganic/organic nanocomposite layer was observed. The investigations are a basis for the understanding and future development of thin conversion films on zinc alloy surfaces. Finally, Scanning Kelvin Probe measurements showed the inhibiting effect of the conversion treatment under the conditions of atmospheric corrosion.     

铝合金表面钛酸盐化学转化膜研究

基于含铬处理对环境影响的考虑,世界范围内都在积极开发有效的替代技术,用于铝合金腐蚀保护从而代替铬酸盐转化处理。研究了铝合金钛酸盐处理技术。通过循环极化曲线和盐雾试验测定,钛酸盐和铬酸盐化学转化膜都有非常宽的钝化区,在NaC l(c=0.5 mol/L)溶液中2种氧化转化膜均没有击穿电位;钛酸盐转化处理的样品经336 h盐雾试验表面无点腐蚀发生。钛酸盐转化膜可以为铝合金提供良好的腐蚀保护作用。     

热浸镀锌层高锰酸盐转化膜工艺

为了进一步提高热浸镀锌层的耐蚀性能,利用化学转化处理方法在热浸镀锌层表面制备了高锰酸盐转化膜.研究了工艺条件对转化膜耐中性盐雾腐蚀性能的影响,确定了最佳工艺为:KMnO<,4>质量浓度20 g/L,处理温度40℃,pH 2.5～3.0,处理时间60 min.采用扫描电镜、能谱、X射线光电子能谱、X射线衍射等技术对转化膜...     

变形铝合金表面锆基化学转化膜的研究现状

化学转化膜是金属表面主要的处理方法之一,具备良好的附着力和耐蚀性,能为铝合金提供一定的临时防护。传统的六价铬酸盐化学转化膜在日渐严苛的环保压力下已经逐渐淘汰,取而代之的是近几年发展迅猛的三价铬及无铬锆基化学转化膜。铝合金可分为铸造铝合金和变形铝合金,按照所含主要合金元素和热处理状态可分为若干个系列和型号。本文选取几种典型的变形铝合金,综述了不同铝合金微观组织对转化膜成膜过程的影响,化学转化液添加剂、预处理和后处理工艺对转化膜性能的调控及作用机理,以及几种典型商业钝化剂在变形铝合金表面的应用。总结了目前变形铝合金表面锆基化学转化膜仍面临的问题和发展趋势,未来化学转化膜需在满足新型铝合金发展要求的基础上,通过不同有机、无机添加剂以及外场作用对转化膜的成膜均一性、完整性进行调控,以提高转化膜的综合性能。     

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.     

ZrO2 sol–gel pre-treatments doped with cerium nitrate for the corrosion protection of AA6060

Sol–gel coatings represent an alternative corrosion protection method to the chromate based systems which must be replaced. Recently, it was shown that ZrO₂ based sol–gel coatings deposited on AA6060 can provide a good corrosion protection to AA6060. However, ZrO₂ based sol–gel systems are not able to provide the self-healing effect which is instead the peculiar property of chromate conversion coatings. The structure of the ZrO₂ based sol–gel systems does not contain species able to restore the barrier properties when defects or damages impair the coating protection.     

溶胶改善镁合金化学转化膜的研究进展

镁合金表面化学处理的方法包括:铬酸盐转化、磷酸盐转化、氟锆酸盐转化、锡酸盐转化、稀土转化等.处理后的转化膜存在孔隙,有微裂纹,需要进行封孔处理,可通过溶胶改善镁合金的转化膜.对涂覆后的化学转化膜进行热处理或采用微弧氧化处理,可以提高溶胶与膜层的结合力与耐蚀性.     

Synergistic effect of potassium metal siliconate on silicate conversion coating for corrosion protection of galvanized steel

Silicate conversion coating is considered as an alternative to hexavalent chromium conversion coating for corrosion protection of galvanized steels. However, the coating must be treated with hot air afterward in order to obtain a water resistant silicate layer and to increase corrosion resistance of the coating. Moreover, it is difficult to apply the uniform thermal treatments on the large irregular shaped metal parts. Furthermore, the heat treatment is an energy consuming process. In this work, the effects of adding potassium methyl siliconate (PMS) into the silicate conversion coating on the anti-corrosion properties were studied. The silicate coating layers were formed by the simple immersion into the silicate solutions and the subsequent ambient drying at room temperature. It was verified that the addition of PMS increased the temporary corrosion protection ability without the post-thermal treatments. Coating thickness was around 100 nm and K ion was distributed preferentially at zinc-silicate interface. It was proved that the more hydrophobic surface was formed mainly due to the substantial decrease of the concentration of K ion at the coating surface by the addition of PMS. It was a methyl group of PMS that changed the hydrophilic silicate layers into hydrophobic ones. Subsequently, the enhanced water barrier property of the hydrophobic silicate layers increased the corrosion resistance of the silicate-coated galvanized steel.