Anodizing
Contents
Introduction
Anodizing is an electrolytic process used to increase the thickness of the natural oxide layer on the surface of metal parts.
Applications of Anodizing [1]
- It increases resistance to wear and corrosion, and provides better adhesion for paint primers and glues than bare metal does.
- Anodic films can also be used for a number of cosmetic effects, through either of the following:
- thick porous coatings that can absorb dyes
- thin transparent coatings which add interference effects to reflected light
- It is also used to prevent galling of threaded components.
Why Anodize? [2]
Black inorganic anodized aluminium alloys are used for passive thermal control on spacecraft and also for avoiding stray light in optical equipment.
Thermal regulation of satellites happens primarily due to radiation, since convection is absent, thanks to the vacuum of space. Thus, satellite temperatures are often passively controlled by choosing materials/surfaces having suitable thermal properties, i.e. having convenient solar absorptance (α) and emittance (ε).
Black coatings have the feature that their α/ε ratio is very near to one, which allows more effective transferring of the heat via radiation.
As mentioned in this paper, "Black anodic films (α > 0.93 ; ε > 0.90) including inorganic dyes are mainly used because of their low cost, their corrosive and wear resistances during storage, as well as the low risk of contaminating the spacecraft’s instruments, especially by outgassing.
The anodic film is developed by electrochemical oxidation of the metal surface, without addition of any substances. From this point of view, the anodic film is not a coating, but a conversion interface tightly bound to the metallic substrate."
A four-digit numerical designation system is used to identify wrought aluminium and aluminium alloys. The first digit of the four-digit designation indicates the group.
Aluminium > 99.00 %—1XXX.
Aluminium alloys grouped by major alloying element(s);
Copper—2XXX;
Manganese—3XXX;
Silicon—4XXX;
Magnesium— 5XXX;
Magnesium and Silicon—6XXX;
Zinc—7XXX;
Other elements—8XXX;
Unused series—9XXX.
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Mechanical Behavior and Defects
Anodizing is an electrochemical conversion process and does not involve the addition of external material. Therefore, anodic films can be considered to be sufficiently adherent for the routine/normal applications. The coloured sealed films are studied from the mechanical point of view, before and after thermal cycling. This cycling simulates the space environment, potentially inducing internal stress, cracking and ultimately flaking of the coatings.
A 2005 alert from the European Space Agency (ESA) mentioned many cases of particle detachment from black anodic films, supported especially on 2XXX and 7XXX type aluminium alloys, after three thermal cycles. Such particle pollution is very dangerous for the lifetime of the satellite.[2]
Cracking
Owing to the different thermal expansion coefficients of the anodic film and the substrate, tensile stresses in the coating are introduced by heating, which can lead to cracking if the fracture limit is reached.
Flaking
Flaking is a major risk than cracking, when it comes to space applications, because the particles released could settle on cold parts like mirrors, lenses or mechanisms.Damage to satellite's instruments could result in decrease in the mission’s lifetime. Also, the thermo-optical properties of the surface could change if the level of flaking is high, affecting thermal control. Flaking is generally observed on parts that had undergone thermal cycling. The loss of adhesion is revealed, for example, when you pull off tapes used to fix thermocouples to the surface during thermal cycles in the Thermo-vacuum test. [2]
Crazing
When the aluminium substrate is deformed in any way, the anodized finish will craze (crack). This happens due to difference in elasticities: the base aluminum has a small degree of elasticity, but the anodized coating has negligible, or practically zero elasticity. When the part is bent, the base aluminum will deflect to an extent without failing, while the coating cracks immediately upon deflection. [3]
Testing
In space, satellites are directly lighted by the sun and then pass into the shadow of Earth, causing thousands of thermal cycles during their lifetime (talking of Earth-orbiting satellites here). To simulate the space environment, the ESA Standard recommends performing 100 cycles between -100 and 100°C under vacuum which lasts for 5 minutes minimum and a have a slope of 10°C per minute. These conditions are defined for the general case and for all kinds of materials and equipment (from polymers to electronics).
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