Anódos alumínium-oxid - fém alkatrészek védelme a korróziótól

Aluminum anodizing has been used for over 90 years to protect metal components from corrosion. The electrochemical process alters the surface chemistry of aluminum to form a dense barrier layer with porous pores for maximum corrosion protection.

Studies have demonstrated that anodizing voltage can control ion migration within a pore-base oxide layer, providing for fast, efficient cell/pore ordering processes.

Korrózióállóság

Anodized aluminum is typically resistant to corrosion; however, as with all metals it will eventually corrode if scratched or dinged and the exposed aluminum becomes exposed – this phenomenon is commonly known as galvanic corrosion or wet install. While galvanic corrosion or wet install may pose problems for industrial applications like aerospace, marine engineering and structural engineering where surfaces may become scratched or dinged frequently, corrosion does not pose as great a threat when surface protection measures such as scratch proof coating are in place.

Anodic alumina’s corrosion resistance depends upon both its morphology and composition of its oxide layer, produced via anodizing. Anodization creates an oxide layer with a very high aspect ratio, consisting of two distinct layers – a porous hydrate on top and an inert barrier layer underneath; its permeability depends upon temperature, electrolyte type and procedure employed during anodization.

Studies have been undertaken to understand how the morphology of an anodic oxide layer can be altered to increase its corrosion resistance. Different anodizing processes were explored such as sulfuric acid anodizing (SAA) and self-ordering anodizing; SAA operates at higher voltages and temperatures than CAA to produce thicker anodic oxide layers with more porous pores; anodic alumina can also be dyed for aesthetic reasons and lubricated using dry film lubricants, Teflon or paint for increased wear resistance and adhesion.

Historically, one of the best ways to enhance anodic aluminum corrosion resistance has been sealing its pore structure. This can be accomplished by immersing anodized aluminum in a solution which reacts with the outer surface of its oxide layer and walls of its pores to form boehmite crystals that fill any resulting gaps and act as a solid barrier between aluminum substrate and its environment.

This method has been tested in various sealing solutions and for various durations, and results showed that as the anodized aluminum was immersed longer into its respective solution, its corrosion current density decreased and concentration of ions within it reduced optimal sealing times.

Elektromos szigetelés

Anodising aluminium components has been in use since the 1920s as an effective means to safeguard them against corrosion. Through electro-chemical oxidation, an anodised surface undergoes chemical change which results in the creation of an extremely hard and wear resistant oxide layer which also acts as an electrical insulator – all achieved without needing additional layers being added onto it.

An aluminium anodic oxide layer can be produced using direct current in an electrolytic solution with an aluminium object acting as the anode. This creates an electric field which induces oxygen release at the anode surface while simultaneously preventing hydrogen ions from entering from the cathode side of the cell, enabling aluminium to create an naturally hardwearing aluminium oxide coating which can then be customized into regular porous structures.

As the anodising voltage is increased, so too does pore formation rate. This is because electric field strength increases at higher voltages and as such ion movement rate is faster in the pore base, leading to runaway conditions whereby the pore base grows much larger than expected. This phenomenon is commonly known as runaway.

High voltages used during anodising not only accelerate oxidation speed, but can also cause the pores walls to become hydrated as ions move within their structure. As a result, these walls typically contain some pure alumina (Al2O3) along with anions from electrolyte solution, water, and small quantities of nanocrystallites [7].

Aluminium anodised in certain acidic media produces a regular self-organizing pore structure which makes for effective electrical insulation, according to The Handbook of Chemistry and Physics 43rd Edition. Alumina boasts the highest dielectric strength among naturally occurring materials.

Hővezető képesség

Due to the increased demand for high-density electronic devices, there has been an urgent need for innovative thermal management materials. As such, studies are underway into creating nano-alumina with enhanced thermal properties for use as liquid thermal interface material, gap fillers or coatings – leading to numerous studies being done into its fabrication and application in liquid thermal interface material, gap fillers or coatings applications.

Anodization produces alumina with various physical properties, including thermal conductivity. Unfortunately, measuring its thermal conductivity can be challenging due to its open space-frame structure; to accurately assess anodized alumina membrane thermal conductivity measurement it is necessary to separate longitudinal from transversal pores channels using photoacoustic technique or effective medium theory (EMT) modeling techniques.

An anodization process begins by applying an electric current to an Al substrate surface through an electrolyte and producing an indented landscape, which serves as the site for pores to form during a subsequent anodization step. Figure 10 schematically depicts how these pores produced during this second anodization step are densely packed with ordered channels running straight and parallel along their surfaces.

An anodized alumina template’s pore diameter can be controlled through chemical etching by widening its pores through chemical etching. This process typically results in the gradual dissolution of oxide layers surrounding its pore channel and thus allows an adjustable channel diameter ranging between 8 nm and 530nm to be achieved.

Thermal conductivity of anodized alumina depends not only on its pore diameter and process type, but also on the substrate morphology – altered by mechanical, thermal, and chemical pre-treatments – and history of its Al substrate, such as having preexisting oxide layers which alter self-ordering of pore structures during two-step anodization process resulting in various values for thermal conductivity reported in literature.

Moisture Resistance

Anodizing increases the thickness of the natural aluminium oxide layer that naturally forms on aluminum parts to produce a thick, resilient and chemically inert coating that lasts far longer than original parts exposed to harsh conditions. Furthermore, anodization makes materials chemically resistant against substances like oxidizing acids that would normally discolor and degrade untreated aluminum; meaning this treatment keeps materials in pristine condition longer despite harsh environments.

Anodized aluminum can also be dyed a variety of colors to produce unique finishes, while dyeing process also improves some natural properties such as its emissivity – making anodized aluminium ideal for radiators and heat exchangers.

Anodizing is also one of the more environmentally friendly metal finishing processes available, unlike integral color anodizing, as it does not use chemicals and does not produce volatile organic compounds (VOCs). Furthermore, unlike electroplating processes which produce heavy metal ions or halogens in their effluent stream; instead their by-products are recycled into products like alum, baking powder, cosmetics and newsprint manufacturing or used as industrial wastewater treatment systems.

Researchers discovered, using a scanning electron microscope, that the wettability of anodic porous alumina (APA) films could be altered by altering their synthesis conditions. Their team created an AAO humidity sensor with high signal intensity, response and recovery time by anodizing commercial 1050 aluminum alloy at 20 V in oxalic acid for one step anodization at 20 V rather than the more traditional two-step anodization at 40 V fabrication method – significantly cheaper and faster fabrication method for AAO humidity sensor formation.

Research also demonstrated that AAO films’ wettability can be enhanced further by changing pore diameter. A gradient was achieved whereby wettability increased from both ends towards the center, where water droplets formed moved along this gradient before merging at once into one large droplet – this method may prove particularly useful when fabricating microfluidic devices or analytical chips.