EIS evolution of anodised aluminium by cerium salt sealing in NaCl solution with exposure time

Abstract

The sealing of porous anodic oxide film on aluminium 2024 involving immersion of the anodised aluminium in an aerated solution containing cerium salt under the application of pulse power at room temperature has been proposed as an alternative to dichromate sealing method. The sealing process shows some advantages. The corrosion behaviours of the anodised aluminium unsealed or sealed by dichromate and cerium salt respectively were investigated at different exposure times up to 30 days in 1M NaCl solution by means of electrochemical impedance spectroscopy, and the damage functions were calculated in this work. The results show that the damage functions of anodised aluminium sealed with cerium salt are lower than those unsealed or sealed with potassium dichromate. Cerium salt sealing provides a better corrosion resistance of anodised aluminium in NaCl solution for long exposure time than dichromate sealing.     

Optimization of the nickel fluoride cold sealing process

This paper investigates the effect of varying sealing parameters, in order to optimize the conditions used to achieve a good seal quality for nickel fluoride cold sealed anodized aluminium. The effect of time, temperature and solution pH on seal quality was studied by British/ISO standards tests such as phosphoric acid/chromic acid, dye absorption and admittance. The quantity of nickel absorbed in the pores of anodic films as a function of solution parameters was measured by Atomic Absorption Spectroscopy (AAS), in order to find out the relationship between seal quality tests and nickel absorption.

Although all the sealing parameters were found to have an effect on sealing quality and quantity of nickel absorbed in the pores, the major effect resulted from temperature which was found to be critical. The best results were found to be at a temperature of ≤ 25°C for a 20μm film. The quantity of nickel absorbed in the pores as shown by AAS, tends to correlate with the results of the seal quality tests.

The effect of post-treatment (final rinse) with hot deionized water (70°C) was also investigated. The post-treatment was found to accelerate the aging of cold sealed finishes and the results of optimized cold sealed samples which have been post-treated are very promising for commercial use.     

Inhibition of localized attack on the aluminium alloy AA 6351 in glycol/water solutions

A previous study had shown that exposure to degraded propylene glycol(PG)/water solution (1:1 in volume) at 80°C for 60 days or to boiling PG/water solution for 30 days induce remarkable localized attack on aluminium alloy AA 6351 (nominal composition: 1% Si, 0.6% Mg, 0.3% Mn, balance Al). In the present work degradation of the solution was achieved by holding the PG/water solution at its boiling temperature for 30 days in contact with AA 6351. The observed localized attack was mainly attributed to the action of copper ions dissolved from the aluminium alloy (copper content = 0.07%) and then deposited as small spots acting as efficient cathodic areas. The objective of this work was to examine the feasibility of enhancing pitting resistance of AA 6351 by adding suitable inhibitors to the solutions. The compounds used were two inorganic salts: sodium molybdate and sodium tungstate and two derivatives of pyrimidine: 2-aminopyrimidine (2AP) and 2-hydroxypyrimidine (2HP). The inhibiting efficiencies of these substances were tested by both short-time electrochemical tests (galvanic coupling tests and polarization curves) and long-time immersions under experimental conditions causing the localized attack. Molybdate, tungstate and, to some extent, also 2AP efficiently inhibit AA 6351 localized corrosion in degraded solutions at 80°C and in pure boiling solutions, for long exposure periods. The short-time electrochemical tests suggest that molybdate and tungstate are able to retard the electrochemical processes occurring on both the aluminium alloy and the small copper cathodic area produced by copper deposition. On the other hand, the 2AP efficiency is attributed to some complexing capability of this pyrimidine derivative towards dissolved copper ions, that are stabilized in solution. 2HP does not prevent AA 6351 localized attack.     

进口铸铝合金件阳极氧化膜的分析

通过电子探针和X射线光电子能谱分析,表明进口铸铝件表面的阳极氧化膜经过减摩封孔处理,其耐磨性明显优于未作减摩封孔处理的国产件膜层.为此,介绍了几种减摩封孔处理方法.     

The Production of Coloured Anodic Films without the use of Dyestuffs

A full investigation has been made into the methods of obtaining coloured anodised aluminium by means of inorganic pigments and by some sealing treatments. The mechanism of absorption of inorganic pigments by the pores of the anodic film is discussed, and it is concluded that to obtain uniform colours it is necessary to control very carefully all the variables in the anodising and colouring techniques.

The majority of the colours described were subjected to both natural exposure and accelerated weathering tests, and some of the inorganic pigments were assessed by direct corrosion of coloured anodised panels in salt water. It was concluded that the most durable, light-fast pigments in the range of the present investigation are copper sulphide,§ ferric oxide, lead chromate, and prussian blue sealed in dichromate.     

Effect of post treatment on nickel fluoride cold sealed finishes

The performance of accelerated aged (post-treatment) nickel fluoride cold sealed finishes is compared with those of room temperature aged and conventional hydrothermal sealed finishes. It was found that accelerated ageing at temperatures of 60 and 80°C for 5–30 minutes in deionized water will improve the results of sealing quality tests of cold sealed anodized finishes, however it does not satisfy all the sealing quality tests applied to conventional hydrothermal sealed films. The results of acid dissolution and dye spot tests are acceptable for post-treated cold sealed films, and in general even better results than conventional hydrothermal sealing can be obtained with various metal salts used as post-treatment. Nevertheless, the admittance test was found to be difficult to satisfy and further ageing of 1–14 days was necessary to pass it. Such a problem was overcome by dual sealing, in which the cold sealed film was treated for a very short time (0.5min/μm) in boiling deionized water, resulting in films with superior properties compared to those sealed by conventional hydrothermal processes.     

Surface Treatments for Enhanced Wear Resistance of Aluminium

This report presents a survey of various surface treatments to improve the wear resistance of aluminium. Over 25 different treatments have been produced and evaluated in laboratory and in application tests. Three plastically formable alloys and one pressure die casting alloy were included.

The different coating systems have been characterised with respect to structure and morphology by scanning and transmission electron microscopy. The laboratory wear tests were performed by using Taber abraser and pin on disc as standard methods. As a comparison, a non-standard reciprocating adhesive wear test set-up was used. In addition, a corrosion test by accelerated outdoor exposure (SCAB) was performed. The application tests were a rotating PTFE sealed sleeve, a station chain in a food packing machine and a yarn feeder.

The results from the abrasive and adhesive laboratory wear tests have been correlated to the results in the application tests. For some of the evaluated coating systems the correlation between the laboratory and the application tests is reasonably good. However, substantial difference in performance was found for other systems.     

Die Kaltverdichtung anodisch erzeugter Oxidschichten des Aluminiums

The cold sealing of anodic oxide layers on aluminium Vacuum treatment of fresh anodic oxide layers on aluminium changes their electrical properties: the capacity is reduced by 75 % while the electric resistance is increased by the factor 10. Subsequent the sealing of anodic oxide layers in water no such changes are produced during vacuum treatment. The latter gives further rise to changes of reactivity and corrosion resistance which become comparable to the respective properties of sealed layers. An interpretation of these effects of vacuum treatment would require further investigations using IR spectroscopy, NMR and determination of the specific surface area.     

溶胶-硬脂酸封闭法对锌与铝阴极间结合力的影响

目的降低电锌与铝阴极间的结合力,提高剥锌效率,研究锌电积过程中锌成核以及结合力的变化情况。方法采用溶胶-硬脂酸封闭法对铝阴极表面进行处理,通过电子扫描显微镜观查铝阴极表面形貌及锌电积初期成核情况,采用万能试验机测试锌铝间结合力,同时结合电沉积实验考察封闭铝板对电流效率的影响。结果封闭铝板表面微孔被封闭,整体光滑平整。与普通铝板相比,其成核率较低,以晶核的长大为主。结合强度呈现出"中心高、四周低"的规律,且结合力随着电积次数的增加而增大。封闭铝板局部结合强度峰值自5.64 kg/cm~2增加到6.34 kg/cm~2,相较普通板结合强度降低了30%左右,在很大程度上降低了剥锌难度。封闭铝板电流效率的平稳性也优于普通铝板。结论采用溶胶-硬脂酸封闭法来降低铝阴极与锌间结合力,降低剥锌难度和提高脱锌效率,切实可行。     

Corrosion behaviour of aircraft coating systems in acetate- and formate-based de-icing chemicals

Corrosion behaviour of four coating systems, which are commonly used in aircraft components; namely cadmium-coated and subsequently chromate-treated steel AISI 4340, aluminium-coated steel AISI 4340, anodised aluminium alloy 7075-T6 and chromate-treated aluminium alloy 7075-T6, were investigated in six commercial acetate- and formate-based de-icing chemicals. The results show that the aluminium-coated steel specimens experience least corrosion among the four coating systems; no corrosion is detected in potassium acetate-based de-icing chemicals (Safeway KA HOT, Safegrip and Safegrip+), although some corrosion is observed in the other three chemicals (Safeway SD, Safeway SF and Meltium). In contrast, the coatings in the other three coating systems are damaged in all six de-icing chemicals. The thickness of the coating does not necessarily predict how well the coating will protect the substrate; the thickest coating, several tens of micrometres, is apparent on the cadmium- and chromate-coated steel specimens, while only a few micrometres thin aluminium coating on the steel substrate is the only coating system that can survive the corrosion tests in some of the de-icing chemicals. Comparison of the results from the two test methods used in this research, polarisation measurements and chemical exposure tests, shows that, for each coating system, the extent of corrosion, as evaluated on the basis of weight changes calculated from corrosion current density values and those experienced during chemical exposure tests, is different: the weight changes that materialise during exposure tests are, at least, from one to two orders of magnitude higher than those predicted by calculations. In this paper, these observations are discussed and explanations for them are presented.     

毛细管铝封无搭边级进冲挤技术

将挤孔——切连皮——落料——空心件正挤压4种工序巧妙地复合在一套模具上，而且模具结构并不复杂，成功地在400kN冲床上实施了该工艺，使空心毛细管铝封片级进挤压工艺生产率达到每分钟60件，大幅度提高了生产效率。本还在落料工序中采用无搭边新技术，一改冲裁落料中有搭边的常规模式，减少了搭边损耗，大大提高了铝板材的利用率。     

Corrosion in solar heating systems. II. Corrosion behaviour of AA 6351 in water/glycol solutions

A research was carried out in order to investigate the corrosion behaviour of the metals most commonly used as construction materials for solar absorber plates. With this view, an attempt was made to test the corrosion resistance of the aluminium alloy AA 6351 (nominal composition: 1% Si, 0.6% Mg, 0.3% Mn, the balance Al) towards common uninhibited heat transfer fluids, such as ethylene and propylene glycol/water mixtures. Long time gravimetric tests consisted in up to 60 day exposures of the aluminium specimens to pure, chloride-polluted, or degraded glycol/water solutions, at the temperature of 80°C. The degradation into acidic products, experienced by heat transfer liquids in service, was simulated by keeping the ethylene and propylene glycol/water solutions at their boiling temperature for 30 days, in contact with copper. In glycol/water solutions the presence of chlorides at low concentration (200 ppm) caused the aluminium corrosion rates to increase by more than one order of magnitude, while in degraded solutions, containing 143 or 86 ppm cupric ions, corrosion rates higher than two order of magnitude with respect to pure solutions were obtained. During the gravimetric tests, pitting corrosion was observed in some cases and its extent was rated by evaluating the deepest and the average metal penetration, the pit density and the average pit size. The influence of heat transfer on the alloy AA 6351 corrosion and on the couple copper/AA 6351 efficiency was evaluated by gravimetric and electrochemical tests. Heat transfer through aluminium was found to significantly increase the aluminium alloy pitting potential. On the contrary, it stimulated the aluminium galvanic corrosion, when applied on either aluminium or copper. Under galvanic coupling conditions, the aluminium corrosion rates calculated from the average galvanic currents were a very little contribution to the gravimetric corrosion rates. This demonstrates that in low conductive solutions the risk of matching such dissimilar metals as copper and aluminium does not reside in the galvanic contact itself, but mainly in the mere presence of the noblest metal in the same solution where aluminium is immersed.     

镁合金表面电弧喷涂铝涂层在氯离子中的腐蚀行为

采用电弧喷涂技术在镁合金AZ91基材表面制备铝涂层.通过5(质量分数,%)NaCl溶液浸泡试验和盐雾试验,考察了镁合金表面铝涂层在氯离子中的耐蚀性能及其腐蚀行为.利用光学显微镜、扫描电子显微镜(SEM)和X射线衍射仪(XRD)分析技术对涂层形貌、显微组织结构和相组成进行了研究.结果表明,封孔处理后的试样耐蚀性能有较大提高,未封孔的涂层试样腐蚀比原始镁合金更严重.     

Vanadate post-treatments of anodised aluminium and AA 2024 T3 alloy for corrosion protection

Porous anodic films formed on aluminium in sulphuric acid were post-treated using various double-dip processes that incorporate a sparingly-soluble, vanadate, corrosion inhibitor. The processes were optimised by investigating the effects of the double-dip order, the rinsing step, the immersion time and the double-dip method. The optimised processes were applied to AA 2024-T3 alloy, which was then exposed to salt spray. V-Ce(III) and V-Zn processes provided corrosion protection similar to that of sealed, sulphuric acid-formed films. A V-Ni process provided increased protection, similar to that of hot water sealed films formed in chromic acid.     

Corrosion in solar heating systems. I. Copper behaviour in water/glycol solutions

A research programme has been developed in order to investigate the corrosion behaviour of metallic materials commonly used in solar heating systems. This paper presents the results of an experimental study on copper corrosion resistance in ethylene and propylene glycol/water solutions (1:1 by volume) constituting the most common bases of heat transfer fluids. Long time gravimetric tests were carried out on electrolytic copper at 80°C, even in glycol/water solutions previously degraded at their boiling temperature or polluted with 200 ppm chlorides. Chemical compositions, semiconducting properties and morphological characteristics of all surface products were investigated by X-ray diffraction analysis, pulse photopotential technique and SEM observations, respectively. Heat transfer effects on copper corrosion and copper/6351 aluminium alloy couple efficiency were evaluated by electrochemical tests. The following results were obtained:

• – Ethylene and propylene glycol/water solutions are low corrosive media. Nevertheless, chloride pollution and/or high temperature degradation of glycols markedly increase their aggressivity. Under all the experimental conditions, copper corrosion rates are higher in ethylene than propylene glycol solutions.
• – In chloride-free solutions, heat transfer stimulates the cathodic reaction of the copper corrosion process.
• – Galvanic contact between copper and aluminium alloy always causes pitting corrosion on aluminium electrodes. The severity of the pitting attack is enhanced by the presence of heat transfer conditions on copper and/or chloride ions in the solutions, particularly in ethylene glycol.

     

Corrosion and corrosion prevention of magnesium alloys

Introduction of high purity alloys improved the corrosion resistance of magnesium alloys significantly. This has led to an increased use of magnesium for components like valve covers, transmission housing, and gear box housing. Because of the unnoble nature of magnesium, galvanic corrosion is the main challenge when magnesium is used in corrosive environment. By proper material selection, proper design and selective use of coatings and insulation materials, the risk for galvanic corrosion is significantly reduced. Test results show that fasteners made of aluminium of the 6000 series reduce galvanic corrosion of magnesium to very low levels in salt spray tests. Combinations of plated steel fasteners and aluminium washers are very efficient for galvanic corrosion prevention. Furthermore, it seems that sealed zinc plating is the best type of plating for steel fasteners. The plating must, however, be free from defects and the bolt head design is an important factor to get a high quality plating.     

Untersuchung der Korrosionsschutzwirkung von Wärmeträgern Teil 2: Chemische Korrosionsuntersuchungen mit dem Rotations-Zylinder und zum Schutz gegen Spaltkorrosion

Testing the corrosion protection of heat-transfer fluids Part 2: Chemical corrosion tests using the rotating cylinder and for the protection against crevice corrosion The second part of the report refers to chemical corrosion tests of the protective effect of inhibited heat-transfer fluids provided with anti-freeze mixtures of water and glycol on metals by means of the rotary cylinder testing method. The metals to be tested were plain steel, cast iron, wrought and cast aluminium alloys, copper materials as well as a solder alloy. Moreover the influence of time, dilution ratio and flow on the corrosion protection of some metals was investigated. Based on the test results of part one and two, standard testing methods and limiting maximum values were defined for the evaluation of the protective effects proved by these testing methods. Finally investigations for the protection against crevice corrosion are described. This publication will end with a third part reporting on electrochemical investigations according to the RZ-testing method.     

Laser roll welding of dissimilar metal joint of titanium to aluminium alloy

Recently, the demand for dissimilar metal joints of titanium to aluminium alloy has arisen in industry, especially in the transportation vehicle industry. However, it is well known that fusion welding of titanium to aluminium alloy is difficult because of generating the brittle intermetallic compound at the joint interface. Therefore, new welding processes with high reliability and productivity for these dissimilar materials are demanded. In the present work, Laser roll welding of titanium to aluminium alloy using a 2 kW fibre laser was tried to investigate the effects of the process parameters on the formation of the interlayer and the mechanical properties of the joint. As a result, the cross-section of the joint shows partial melting of the aluminium sheet and spreading of molten aluminium alloy on the titanium sheet occurs during the welding thermal cycle. Various types of intermetallic compound were confirmed at the interlayer of the welded joint. The specimen with a bonding width of 2.8 mm failed in the base metal of titanium in the tensile shear test. In Erichsen cupping tests, the Erichsen value was 5.7 mm. This value was 89% of the base metal of aluminium sheet.     

Ten-Year Exposure Tests on Anodized Aluminium in a Severe Industrial Atmosphere

The paper describes the results of examination of specimens after 10 years’ exposure to a very severe industrial atmosphere. The work is a continuation of tests previously reported by Whittaker and Kape. All anodized films show signs of pitting and the specimens have lost reflectivity to a marked extent. Resistance improves with increasing film thicknesses and higher purity of the aluminium. Periodic washing with distilled water does not decrease pitting or delay its onset.     

Aluminium in fresh waters

Abstract

The paper summarises research at these Laboratories on corrosion of aluminium in fresh waters. The type of corrosion affecting aluminium depends on water composition. Deep pitting requires oxygen, chloride, carbonate hardness and copper, but the higher chloride contents encourage more general corrosion. Higher carbonate hardness reduces the number of pits formed; sulphates have a similar effect and also increase pit depth. Except in waters of very high copper content, attack is generally less at elevated temperatures. Different alloys differ only slightly in resistance to attack. On superpurity aluminium, pits are slow to form but subsequently penetrate at the same rate as in SIC type. The best form of protection for aluminium is cladding with aluminium– l¼% zinc. This prevents pitting of N3 sheet or tube even in highly corrosive hard waters. Spraying with aluminium–zinc or anodising is also effective. Treatment to form boehmite films gives some protection but is ineffective in the more corrosive waters.