3.3 Most common methods and technologies for synthesizing smart coatings
3.3.1 Chemical conversion coatings
The corrosion of metals is one of the main destructive processes that lead to huge economic losses. Polymer coating systems are normally applied to a metal surface to provide a dense barrier against the environmental species in order to protect metal structures from corrosive attack. When the barrier is damaged and the corrosive agents penetrate the metal surface, the coating system cannot stop the corrosion process. The most effective solution so far for the initial combat for active protection of metals to improve the corrosion protection of the metals and alloys is to employ chromate-containing conversion coatings.
Chromate has been used since the early 1900s as a way of controlling the corrosion of active metals. It is the most common type of conversion coating applied to improve the corrosion resistance of many ferrous and nonferrous metals and their alloys. Major reasons for the widespread use of chromating are its self-healing nature, the ease of application, high electric conductivity and high effi ciency/cost ratio. These advantages have made it a ‘standard’
method of corrosion protection. Chromate has also been considered as the
‘pioneer’ smart coating due to its outstanding self-healing capabilities to repair damage and corrosion in several metals and alloys. Self-healing or active corrosion protection (ACP) involves the release of chromate from the coating, transport through solution and action at the site of damage namely pits or micro-cracks.
However, since 1982, the Environmental Protection Agency (EPA) has increasingly limited the use of chromates and other chromium-containing compounds due to their carcinogenic effects. Unfortunately, the same properties that make hexavalent chromates superior corrosion inhibitors (reactivity and oxidizing power) also make them environmentally unsafe.
Hexavalent chromates do not directly react with human deoxyribonucleic acid (DNA). However, just as it is reduced at the corrosion site, the hexavalent chromate is reduced to pentavalent chromate, which is responsible for DNA damage and cancer – the main reasons for legislation against chromate – driving the search for chrome-free ‘green coatings’
that comply with environmental and health legislation. Offi cially, chromate has largely been eliminated in most industrial applications except for some high-performance strategic applications such as in the aerospace industry.
Several strategies have been explored to develop less toxic or more eco- friendly options with self-healing capabilities in order to comply with environmental legislation. During the last decade, several ‘green coating’
schemes have been proposed to improve the corrosion resistance of metals and alloys. However, most of the existing methods are frequently either expensive or unable to produce the surface properties desired for many applications where such metals and alloys would otherwise be highly industrially competitive. The development of active corrosion protection systems for aluminum, magnesium and steel alloys and composite materials is an issue of prime importance for many strategic industries such as automotive, aerospace and petroleum pipelines.
Recently, several authors have successfully presented different new contributions to the development of new protective systems with self- healing ability. The self-healing property of ACP is imparted to the coatings by treating and/or modifying the surface of the metallic substrates with a dilute salt solution of environmentally acceptable salts like cerate, stannate, vanadate, permanganate, silicate, zirconate or molybdate [3–26] . The
proposed coatings would have a self-healing ability and ease of application at low cost and safety.
The healing concept in these systems appears to be that these metals exhibit soluble high oxidation state forms and low oxidation state forms with a lower solubility. When high oxidation state oxides are introduced due to external action such as corrosion, mechanical damage or scratch, they can be dissolved by a contacting solution, transported to defect sites on bare substrate (aluminum, magnesium, steel or composite materials) samples where they are reduced and precipitated to inhibit further corrosion. Results confi rmed that self-healing is possible with conversion coatings other than those based on toxic hexavalent chromates and proved that when they are applied prior to an organic top coating (such as commercial fl uoropolymer, epoxy, etc.), the coatings outperformed the commercially available coatings in electrochemical corrosion-resistance tests and exceed 2000 hours in the standard ASTM B117 salt spray test which enable them to be used in many industrial applications where active corrosion protection of materials is required [3–13] .
Newly developed magnesium-based alloys such as Mg ZE41,Mg AZ31 HP-O and AV31A T6 have attracted the attention of the scientists in academia and industry due to their excellent mechanical properties and light weight. Unfortunately, magnesium alloys have high susceptibility to corrosion. Therefore, novel self-healing surface treatment processes based on chrome-free salts such as ceria, zirconia, stannate, cerate and vanadia conversion coatings have been successfully designed [20–26] .
The new surface treatments can produce a functionally graded coating that will provide covalent bonding for strong coating adhesion and act as a barrier coating to limit the transport of water to the surface of the alloy.
Ideally, the perfect conversion coating should be uniformly distributed over the substrate surface, has low environmental reactivity, good mechanical properties and good adhesion, as well as being environmentally acceptable, industrially applicable and cost-effective.
The key innovation proposed in the new surface conversion coating treatment processes is the development of a novel non-chrome corrosion inhibiting pretreatment technology that is expected to impart self-healing behavior to magnesium substrate. The coating was designed to respond to the electrochemical processes responsible for corrosion by providing a self- healing function to automatically heal the scratch or damage by forming protective oxides [20–26] . The expected broader impact of these fi ndings will mainly be in the automotive and aerospace applications, where the use of lighter-weight magnesium alloys (as compared to the currently used aluminum alloys or steels) will help reduce CO 2 emissions from the exhaust.
Currently some automotive components such as transmission housing and
steering column are made of magnesium alloys, but the new fi ndings will permit the use of magnesium alloys for other components, such as engine blocks, gear boxes, clutch housings and engine cradles. This technology has also been tested on aluminum alloys and composite materials and was found to improve corrosion resistance and add self-healing functionality.