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Mittwoch, 25 August 2021 11:59

Black Electrochemical Coatings for aerospace and allied Applications - Part 3 – Black Anodic Oxide Coatings

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Geschätzte Lesezeit: 15 - 30 Minuten

Anodic oxide coatings are formed on the metallic job by an electrochemical process known as anodizing. The process is carried out in an electrolytic cell where a job is made of an anode and a suitable inert metal as a cathode (Figure 3.1). When the electric current of sufficient voltage is passed through a suitable electrolyte the metal surface is converted to an adherent oxide coating which is an integral part of the substrate [1–4].

Fig. 3.1: Anodizing Process Set-up Fig. 3.1: Anodizing Process Set-up The anodic oxide coating consists of two layers, the inner thin and dense barrier layer, and the more structured thick porous top layer. Metal anodization has been broadly used in industry as a surface treatment technique to render materials with resistance against uncontrolled oxidation, abrasion, and corrosion. Although this technique has been developed for a long time, it was until the 1990s that researchers discovered that highly ordered nano porous structures can be achieved by properly tuning anodization conditions including electrolyte composition and concentration, temperature, as well as anodization voltage. Among all valve metals that can be anodized, aluminium in particular can be anodized into nano porous structures with well-controlled diameter, pitch, and depth [5].

3.1 Applications of Anodic Oxide Film

Anodization on aluminium and its alloys has immense commercial application ranging from household items to aerospace, automobile and defense sectors [3].

  1. To provide decorative finishing. Thick anodic coatings due to their inherent porous nature can be dyed in a wide range of attractive colours.
  2. To protect the metal from atmospheric corrosion. Anodizing provides a uniform stable protective film on the article.
  3. To yield a good base for paints, lubricants and adhesives. Owing to specific texture and porous nature, anodic coating imparts a good bonding characteristic for paints, lubricants, adhesives and a variety of such other materials.
  4. To improve hardness and wear resistance. Anodic film is harder than bare metal, it helps in reducing the friction on the moving parts. Hard-anodizing is a prime requirement for reducing abrasion on sliding surfaces. Hard coats also act as a base for retention of lubricants. As aluminium has a severe tendency to gall, lack of lubricity poses serious problems in applications involving the contact of sliding surfaces in various forming applications.
  5. To bestow thermal and electrical insulation. Anodic film (alumina) is a ceramic oxide coating, it provides excellent thermal and electrical insulation properties. The anodic coating has thermal conductivity of about one-tenth of aluminium and its melting temperature is around 2050 °C far greater than aluminium. The components with thick and hard anodic film provide excellent heat resistance properties and can withstand unusually high temperatures for a short duration.
  6. To prevent cold welding. Hard-anodic coatings are employed to prevent cold welding in space conditions. Cold welding is the condition of jamming of two soft surfaces in contact due to atomic diffusion without application of heat.
  7. To impart good heat radiation properties. The aluminium oxide layer acts as an effective radiator. A polished aluminium metal surface has an emittance value as low as 0.03 which can be significantly increased by anodization. A 10–15 μm thick anodic film developed with sulphuric acid process provides IR emissivity of 0.77 whereas its solar absorptance remains around 0.32. The anodic film therefore acts as an excellent solar reflector for thermal control applications.
  8. To make dielectric films for electrolytic capacitors. In addition of the above applications of thick anodic films, the thin barrier type anodic oxide films are widely used to make electrolytic capacitors in microelectronic integrated circuit technology.

In addition to various industrial usages, micro/nano-porous metal oxides, particularly anodic aluminium oxide (AAO), have a wide and varied range of applications in optical, electronic, electrochemical, and many other engineering devices [6–13]. Due to the unique optical and electrochemical properties, large surface area, tunable properties, and high thermal stability, nano porous anodic aluminium oxide (AAO) has become one of the most popular materials with a large potential to develop emerging applications in numerous areas, including biosensors, desalination, high-risk pollutants detection, capacitors, solar cell devices, photonic crystals, template-assisted fabrication of nanostructures, and so on [14–16]. The structure and geometric features of typical self-organized nano porous anodic alumina are represented in Figure 3.2 [17]. One of the interesting features of the aluminium anodization is its easily scalable low-cost fabrication [18] of uniform and widely controllable pore diameters of AAO films with well-defined geometries [19, 20]. AAO structures can be further engineered into a number of variants via proper combination of wet chemical etching and anodizing processes. These variants include nano wells [21], inverted nano cones [22, 23], nano bowls [24], nanospikes [25–27], and the integrated nanopillar-nano well structures [28].

Fig. 3.2:  Self-organized nanoporous  anodic alumina:  Structure and geometric  features  (Lp = pore length,  dp = pore diameter,  dint = interpore distance [17]Fig. 3.2: Self-organized nanoporous anodic alumina: Structure and geometric features (Lp = pore length, dp = pore diameter, dint = interpore distance [17]

Typically, AAO membranes are quasi-transparent and have well-arranged periodic air pores. A self-assembled AAO porous layer has been widely used as the template for the growth of uniform, periodical, and well-aligned nanotubes and nanowires [29] and the formation of nanoparticle arrays [30], the basis for photonic bandgap structures [31, 32], and the anti-reflection coating for light trapping [33]. For certain applications, which require high optical absorption, lossy colloidal nanoparticles, e.g., metallic nanospheres [6, 34] or carbon nanotubes [35], are typically deposited onto the AAO template for enhancing the light absorption.

A nearly perfect absorber mirror-backed nano porous alumina coating for thermoelectronics and thermophotovoltaics was reported by Farhat et al. [36]. By electrochemically anodizing the disordered multicomponent aluminium and properly tailoring the thickness and air-filling fraction of nano porous alumina, according to the Maxwell-Garnett mixture theory an absorption larger than 93 % over a wide wavelength range from near-infrared to ultraviolet light, i.e., 250–2500 nm can be achieved. This simple approach does not require any lithography, nano-mixture deposition, pre- and post-treatment. The large-area and low-cost dark alumina shows a promising potential for various energy harvesting and conversion applications.

3.2 Anodizing Process

Porous type thick anodic oxide films can be accomplished with slightly soluble electrolytes such as sulphuric, phosphoric, chromic and oxalic acid. While the ‘barrier type’ thin aluminium oxide films (typically < 0.5 μm) can be formed in weak electrolytes (e.g., neutral boric acid, ammonium borate or tartrate aqueous solutions) where the coating is almost insoluble and non-porous. The detailed processes are described elsewhere [1–3]. Sulphuric acid anodizing is the most versatile and widely used anodizing process on aluminium and its alloys. The following typical process sequence can be adopted.

The jobs are first chemically cleaned using a sequence of (a) solvent degreasing in trichloroethylene or other suitable solvent; (b) alkaline cleaning at 55 to 60 °C for 3 to 5 minutes in a solution containing 100 g/L sodium hydroxide, 20 g/L sodium fluoride, 2 g/L sodium polyphosphate, and 1–2 g/L wetting agent; (c) Neutralizing/de-smutting at room temperature in 30–50 % nitric acid solution for 30 seconds for copper containing alloys or in a mixture of acid solution containing 10 ml/ L sulphuric acid, 12 m/L hydrofluoric acid and 25 ml/L nitric acid for 2 to 3 minutes for high silicon and other alloys; followed by water rinsing. The chemically cleaned jobs are processed for anodizing as follows [3, 37, 38]:

(d) Anodizing (Electrochemical Process)

Sulphuric acid, ml/L 120–200

Temperature, °C 15–25

Current density, A/dm2 1.0–2.0

Voltage, V 10–20

Time, minutes 15–45

Appearance transparent

Coating thickness, µm 2.5–25

Microhardness, HV 80–120

(e) Black dyeing with organic or inorganic dye stuff

(f) Sealing of anodic oxide pores in boiling demineralized water (pH 6.5–7.0, 96–100 ºC), for a period of 2–3 minutes / µm of film thickness. Additives like 5 g/L nickel or cobalt acetate or 50 g/L sodium or potassium dichromates may be added in water to reduce the sealing time and/or to improve the sealing quality in particular for marine applications. The salts are absorbed into the coating where they are hydrolysed and precipitated as hydroxides. The process is also found effective to reduce the losses of colour after dyeing.

The pore sealing produces a chemical change in the coating by converting it from aluminium oxide to boehmite [Al2O3.H2O], a stable oxide hydrate. In the course of this change, the coating swells and pores are closed. Sealing improves the corrosion protection of the film but lowers its hardness, abrasion and wear resistance. The adhesion of paints and lubricants is also reduced. Therefore, the anodic coating where wear resistance is to be maintained should not be dyed or sealed.

Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer. Harder and thicker films tend to be produced by more dilute solutions at lower temperatures with higher voltages and currents. Alternating current can be used, but has found no specific advantage. Pulsed current has gained importance in producing the hard-anodic film, where coatings can be obtained at high operating voltages.

When the electrical current is passed, the electrolyte starts to decompose, hydrogen is produced at the cathode and negatively charged O2- are attracted to the aluminium anode. The electrical charge in the circuit causes positively charged aluminium ions to tend to move towards the cathode. However, at the anode surface, the aluminium anions combine with oxygen cations to form aluminium oxide. Initially a thin barrier layer is formed whose thickness is proportional to the initial applied voltage. Once this limited thickness is approached the acid starts dissolving the oxide film, producing small ‘holes’ or pores in it, which are separated from the metal by a thin barrier layer. The diameter of the pores, the interpore distance and the thickness of the barrier layer at the base of the film are usually proportional to the anodizing potential (~1, 2.5 and 1 nm/V, respectively). The pores allow the electrolyte solution and current to reach the aluminium substrate and continue growing the coating. Since there is an electric field associated with each pore, the pores are equidistant. The film is made up of a very large number of small hexagonal shaped oxide cells, some millions to the square inch.

Fig. 3.3: Different colouring systems used for anodic oxide films Fig. 3.3: Different colouring systems used for anodic oxide films The conventional anodic oxide films can be made black coloured by various methods, anodizing and dyeing with organic dyestuff or inorganic pigments; anodizing followed by electrolytic colouring and integral colour anodizing. The colouring has to proceed after anodizing. If the anodized job is left for a long time, the natural sealing of pores may deter the dye absorption. The colour is mainly due to absorption, though optical interference and light scattering may also be responsible for colouring. Optical interference can occur when a thin film of translucent material is present on the surface of a bulk material which is opaque or of a different refractive index. The different colouring systems used on anodic oxide films are represented in Figure 3.3 and are further explained in the subsequent sections.

3.3 Anodizing-Black dyeing

Colouring of anodic film with organic dye stuff is the simplest and most widely used process for decorative finishes. The porous anodized aluminium is immersed in a dye solution, and over time, dye is absorbed into the pores. The colour range and artistic versatility are endless [39, 40]. Dye is distributed throughout the coating near the top surface. The dye absorption can be improved by increasing the porosity (pore size) of the film. However, the higher pore size is detrimental to corrosion protection and optical stability of the coating. Every dye has its own colour fastness characteristics, but all fade to some degree because of bond cleavage within the organic molecule. This cleavage can occur as a result of UV irradiation, chemical oxidation or combination of both. The organic dye stuff also degrades at elevated temperatures (> 65 °C).

In addition to dyeing with organic dye stuff, two inorganic colouring techniques are used to acquire the coloured finish on anodized aluminium, viz., adsorption and electro-deposition of metal-containing pigments. The absorption of inorganic colouring materials follows the mechanism similar to organic dye stuff and improves with increasing the pore size and volume.

3.4 Anodizing-Inorganic Black Colouring

A process of inorganic black colouring of anodic oxide film for space applications has been described by Sharma et al. [38]. After obtaining an anodic film thickness of 25 ± 5 µm by sulphuric acid anodizing, the black colouring was carried out by immersing the samples in the solutions of

(a) Cobalt acetate, 150–300 g/L, for 15 minutes, followed by water rinsing and

(b) Yellow ammonium sulphide, 20–40 g/L for 10 minutes; water rinsing.

The colouring steps (a) and (b) may be repeated to obtain desired depth colour shades. Sealing of pores was carried out in boiling water (>97 °C) for 15 to 20 minutes. The pH of sealing water was maintained within 6.5 to 7.0. Finally, the samples were air dried and the sealing bloom was removed by gently rubbing with cloth.

The black colour in the anodic film is produced due to the precipitation of black metal sulphide in the pores of the film. In the first colouring step cobalt acetate solution is absorbed in the porous structure. When the film is immersed in the ammonium sulphide solution, cobalt sulphide precipitates inside the pores of the film. The precipitated cobalt sulphide is retained inside the pores because its particle size is large enough to restrict its movement outside the pores. Thereafter, the pore sealing is accomplished in hot distilled water.

Co(CH3COO)2 + (NH4)2S → CoS ↓ + 2CH3COONH4

In another method, the inorganic black colouring was carried out with nickel salts [41]. The anodized job was immersed in two successive solutions of

(a) Nickel sulphate 30 g/L and ammonium sulphate 30 g/L at pH 9, for 30 seconds, water rinsing

(b) Sodium sulphide 10 g/L at pH 9, for 30 seconds, followed by water rinsing.

The colouring step was repeated for ~10 minutes in each solution to obtain the desired depth of colour. A solar absorptance of ~ 0.97 and a thermal emittance of ~ 0.90 under optimal conditions can be achieved by these processes with excellent environmental stability.

3.5 Anodizing-Electrolytic Colouring

Electro-deposition of metallic pigments (electrolytic colouring) can be achieved by immersing the anodized aluminium in a metal salt solution and then applying an AC potential. During the cathodic half cycle, particles of metal are deposited at the bottom of the pores in the interphase between barrier and crystalline layer. During the anodic half cycle, the ultra-thin barrier layer oxide separating the porous coating from the aluminium is given time to stabilize. Over time, the metal particles get filled at the bottom of the pores and the resulting colour deepens. Although limited in colour range, this technique is widely used around the world to produce bronze to black finishes for exterior architectural components. Some typical electrolytic black colouring solutions based on cobalt and tin salts as shown in Table 3.1.

Tab. 3.1: Some typical solutions for electrolytic colouring of anodic film   

Bath composition

Voltage (AC)

Time

Colour

Cobalt sulphate, 15–25 g/L Boric acid, 20–30 g/L Ammonium sulphate, 10–20 g/L

12–18 V

8–12 minutes

8–12 minutes

Stannous chloride or sulphate, 10–20 ml/L Sulphuric acid, 10–40 g/L Phenol sulphonic acid, 5–15 ml/L

15–30 V

5–15 minutes

Bronze-Black

The electrolytic colouring baths operate at the ambient temperatures. Unlike anodizing, the process is controlled by voltage and time rather than by current density. The colour is mainly dependent on the amount of metal deposited, for example: 200 mg/m2 tin produces a light bronze colour while 2000 mg/m2 tin a deep black. The counter electrode is usually graphite or stainless steel, although cobalt and tin can also be employed when the bath contains the salt of the corresponding metal.

The anodizing and electrolytic colouring process delivers a stable black anodic film with excellent thermal emittance. However, the process is highly sensitive to operating conditions and the composition of the colouring electrolyte. In practice, sometimes it becomes extremely difficult to obtain uniform colour finishes on complex geometry components.

3.6 Integral Colour Anodizing

The coloured anodic films can also be obtained during the anodizing process itself. The process is called integral colour anodizing as here the colour becomes the integral part of coating. These processes often require precise substrate composition and specifically formulated electrolytes usually containing organic sulfo-acids and low contents of sulphuric acid to produce a series of bronze to black shades.

Integral colour anodizing produces stable coating, but there is difficulty in obtaining an acceptable colour match as the process is highly sensitive to alloy composition and temper [2, 3]. An integral black anodizing process developed by Philips Electronic & Associated was investigated further by Sharma et al. [42] to enhance the thermal emittance of the coating. The following bath formulation and process conditions were found optimal: oxalic acid, 100 g/L, chromic acid 20 g/L and sulphuric acid 5 g/L; pH 1.0; temperature 25–30 °C; current density 2.5 A/dm2; processing time 45 minutes. However, with a minimum coating thickness of 30–32 µm, a maximum solar absorptance of 0.80, 0.82, 0.90 and a thermal emittance of 0.82, 0.83 and 0.85 could only be achieved for AA1100, 2024 and 6061 alloys, respectively.

Franco et al. [43] have investigated the electrochemical behaviour of three black anodic coatings: black dyeing, inorganic colouring, and electrolytic colouring on AA6061 alloys. Anodizing was carried out in sulphuric acid (15 % by weight) at 25 ± 5 °C, and 18 V for a duration of 20–50 minutes. The black coatings were then obtained as per Table 3.2. Post colouring, the hydrothermal sealing was performed by immersing the samples in boiling demineralised water for 30 minutes.

Tab. 3.2: Black anodizing process parametersTab. 3.2: Black anodizing process parameters

Process

Concentrations

Process parameters

Black dyeing

Jet Black, 11 g/L (Commercial organic black dye Khatau Valabhdas, Gujarat, India)

pH: 5 ± 0.5 Temperature: 75 ± 5 °C Time: 15 minutes

Inorganic colouring

Step 1: Cobalt acetate, 200 g/L Step 2: Ammonium sulphide, 25 ml/L

Temperature: 25 ± 5 °C, Time: 15 minutes Temperature: 25 ± 5 °C, Time: 15 minutes

Electrolytic colouring

Sulphuric acid,10 ml/ L Stannous chloride, 20 g/L Phenol sulphonic acid,10 ml/L

pH: 1.1 ± 0.3 Temperature: 25 ± 5 °C Voltage: ~20 V, AC Time: 15 minutes

A comparison of the impedance spectra of all coatings showed that the porous layer resistance is found to be higher for inorganic colouring as compared to plain anodizing and black dyeing. This might be due to the higher stability of the inorganic pigments compared to other techniques. The Rp values (resistance by porous layer) of black dyed sample was found to be lower compared to other colouring techniques which are essentially attributed to the instability of black-dye pigments towards corrosive attack. The electrolytic colouring showed a unique electrochemical behaviour that could be ascribed to the corrosion of the outer thin layer of metallic tin. Linear polarisation studies confirmed that there is no significant difference in the Icorr (corrosion current) values of different coatings although inorganic colouring showed the lowest value.

3.7 Hard-anodizing

Hard anodizing is a term used to produce thick (> 25 µm) and hard (> 200 HV) grey to black colour anodic film on aluminium. Hard anodic oxide coatings find application in the engineering components, where a high wear resistant surface is required such as pistons, cylinders, hydraulic gear and a variety of other moving parts. Thick hard anodic film provides both higher solar absorptance and high infrared emittance values in the range of 0.85–0.90 [44, 45].

The dense hard-anodic coatings are produced using special anodizing conditions, dilute electrolytes operating at low temperature with high current density. The conventional process employs a sub-zero temperature of electrolyte to reduce its solvency action on the coating to build the higher coating thickness. At the sub-zero temperatures the effective conductivity of the electrolyte is reduced substantially and therefore a vigorous electrolyte / job agitation is required to dissipate the heat generated at the coating-electrolyte interface. As the coating formed is insulating in nature, when the process proceeds at a constant high current density, a surge in voltage towards the end of anodizing process sometimes may lead to sparking, i.e., burning of the job.

Pulse hard anodizing provides improved surface morphology and micro hardness. In pulse hard anodizing, the current is pulsed many times a second from base to peak and also periodically reversed. The baseline recovery during the reverse cycle reduces the Joules’ heat at coating electrolyte interface thereby, the bath can operate at the higher current densities at the significantly lower voltages. Also, during the reverse cycle, the liberated hydrogen at the bottom of pores cools the job surface. Thus, the cooling requirement of electrolyte to sub-zero temperatures is eliminated. This leads to an appreciable energy saving. Further, as the bath operates at higher current densities, the same coating thickness can be achieved at a much lesser processing time.

Rajendra et al. [46] have carried out detailed studies on conventional and pulse hard-anodizing. The process details are shown in Table 3.3. For pulse hard-anodizing square wave pulse current having a duty cycle of 60–75 % with a forward frequency of 40 Hz and a reverse frequency of 400 Hz was employed.

Tab. 3.3: Hard-anodizing process parameters

Process parameters

Conventional hard-anodizing

Pulse hard-anodizing

Electrolyte

Sulphuric acid (SG 1.84) 184 g/L and oxalic acid 15 g/L

Temperature, °C

-10 to 0

25 ± 2

Current density, A/dm2

3.25–4.25

6.0–7.0

Voltage, V

24–90

14–20

Time, minutes

80–120

40–50

Thickness, µm

60 ± 10

60 ± 10

Microhardness, HV

250–350

>350

An interesting study on the influence of Fe-bearing particles and nature of electrolyte on hard anodizing behaviour of AA-7075 extrusion products was carried out by Mukhopadhyay and Sharma [11, 12]. These extrusions contained the Al12(FeMn)3Si-based constituent particles, which do not dissolve during anodizing and locally inhibit the growth of anodic film. A greater damage to the growth of anodic film is caused by the alignment of these particles in the direction normal to the growth of anodic film. The authors have demonstrated that the modified strong electrolytes (e.g., addition of a little quantity of hydrochloric acid (4 ml/L of 36 % v/v concentration), nitric acid (7.5 ml/L of 69 % v/v concentration) or ammonium nitrate (30 g/L) to the conventional sulphuric acid-oxalic acid electrolyte) are capable of dissolving the stringer particles during anodization process providing a continuous film growth. The extent of success of this electrolyte system depends on the size of Al12(FeMn)3Si-based particles and their dissolution rate during the course of anodic film formation. A solar absorptance of 0.85-0.92 and IR emittance of 0.85-0.89 was achieved with 50-85 µm thick hard-anodic films.

Franco et al. [47] have prepared the hard-anodic coatings on AA 6061 by three different methods viz., conventional hard anodizing (C-HA), pulse hard anodizing (P-HA) and reverse pulse hard anodizing (RP-HA) using sulphuric acid (100 ml/L) and oxalic acid (15 g/L) electrolyte (Tab. 3.4). The processing time was varied to get a coating thickness of 60 ± 3 μm from all the methods and their corrosion behaviour was compared using electrochemical impedance spectroscopy (EIS) and linear polarisation. The studies indicate that the corrosion resistance of P-HA and RP-HA, which are processed at lower voltage and at relatively higher temperatures, are comparable to that of C-HA which is processed at sub-zero temperatures and high operating voltages. The corrosion resistance of all coatings was found to improve after hydrothermal sealing.

Tab. 3.4: Processing parameters of the three different hard-anodizing methods

Process

Temperature (°C)

Current density (A/dm2)

Duty cycle (%)

Frequency (Hz)

Time (minutes)

C-HA

-5 ± 2

3.25

80-100

P-HA

10 ± 2

5.40

60–75

5–10

50–60

RP-HA

25 ± 2

6.45

60–80 (forward) 60–80 (reverse)

10–20 (forward) 400–500 (reverse)

40–60

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  • Ausgabe: 8
  • Jahr: 2021
  • Autoren: Dr. Anand Kumar Sharma

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