Galvanic Corrosion is an accelerated form of corrosion that occurs when two dissimilar metals are in an electrical contact. The more noble metal drives the corrosion of the active metal and this can be a very fast process. For example if an aluminum frame is connected with steel bolts then aluminum rapidly corrodes and after a few months the whole construction may collapse. So how to prevent galvanic corrosion? First, one should connect only those materials that have a similar electrochemical activity. Second, dielectric corrosion resistant coatings should be applied on the metal parts so that the electrochemical processes cannot take place.
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Graphene – a One Atom Thick Corrosion Resistant Coating?
Graphene – a novel material, that consists of only a single layer of oriented carbon atoms. This material is hundreds times stronger than steel and conducts heat and electricity with exceptional efficiency. A perfect sheet of graphene could actually be so strong that it could lift even a kitten! But can this one atom thick material also be the thinnest corrosion resistant coating ever made? There have been several papers over the last decade discussing this possibility and this peaked our interest. So we first prepared graphene on larger copper substrates by chemical vapor deposition (CVD). Following Raman spectroscopy and scanning electron microscopy (SEM) studies confirmed, that we had a single layer of high quality graphene, that coated the whole substrate. Now it was time to test how well this single atom thick coating can protect the underlying copper from corrosion – for that we used chemical and electrochemical methods. According to electrochemical tests, graphene actually seemed to slow down the corrosion, but additional high resolution imaging with the scanning electron microscope revealed the terrifying truth!
Graphene had started to delaminate and the areas with exposed copper had suffered severe corrosion, unlike bare copper substrates, that corroded uniformly. This severe type of copper corrosion in the defects of graphene was actually caused by graphene itself because of localized galvanic corrosion. In this galvanic couple the large area of highly conductive graphene served as a cathode and the small area of exposed copper as an anode. As a result the copper was continuously stripped from electrons, causing it to easily react with the corrosive environment. So basically it was clear that graphene may initially seem to be a corrosion resistant barrier, but eventually the graphene coated substrate would suffer much more damage than the uncoated substrate! Also, it is impossible to create a large area graphene that has no defects where chloride ions wouldnt be able to slip through. However, it was also true that if the defects in graphene were to be fixed, the galvanic couple and corrosion would be undone. In our laboratory we sealed these small defects in graphene by electrodeposition of polypyrrole. Surprisingly this polymer deposited extremely well on such defects, sealing even holes that were several microns wide. As a result we obtained a nanocomposite coating, that consisted of graphene and polypyrrole. This coating performed well both in chemical and electrochemical corrosion tests.
So what did we learn from all this? The first thing that we learned was the fact that galvanic corrosion can effectively be stopped even at nano-scale, by blocking one of its driving reactions. Second, we discovered that the electrochemical behavior of the defects of graphene is completely different from the defect free area. Third, we realised that polypyrrole may even visualize the quality of graphene for the naked eye because polypyrrole deposits only on graphene and its defects but not on copper if the defects were too big. For example if one built a CVD (chemical vapour deposition) reactor for the preparation of 1 square meter of graphene on a copper foil, then there is a need to somehow see if the foil was actually evenly coated by graphene. By depositing polypyrrole on the graphene coated copper substrate, it is possible to see even with the naked eye if the whole substrate turns black or of there are brigher areas. If there were brighter areas on this copper foil with graphene and polypyrrole, then this would mean that in those regions we dont actually have graphene where polypyrrole could deposit. This would mean that either the gas flow or temperature in the CVD process had not been ideal.
Although things get complicated in the nano-scaled world, the processes are still governed by a set of rules. One day we’ll figure them all out and then we can truly play „dice“ at the atomic scale!
Atomic Layer Deposition – A Method for Making Ultra-Thin Invisible Corrosion Resistant Coatings
Ever thought how nice it would be if the replicator (synthesizer) from Star Trek actually existed? Assembling different materials in an atomic scale however, has been possible already for decades! This exciting technique is called „Atomic Layer Deposition“ (ALD). The deposition process is carried out in a specially designed ALD reactor, where different chemicals enter the reaction chamber one at a time and react with the substrates surface in a self limiting manner. With each deposition cycle a thin layer is deposited and by repeating the cycle thicker material layers can be obtained. An easy example would be the deposition of titanium dioxide by using titanium(IV) chloride and water as reacting chemicals (precursors) and nitrogen as carrier gas.
Although this method is not suitable for creating macroscopic objects, it can be used to significantly enhance their properties such as corrosion resistance, wetting (self cleaning surfaces) or even biocompatibility (brain chips). This method is also used in the production of some solar cells, microelectronic devices and nanostructures. The huge benefit of this method is the possibility to apply films with well defined thickness and composition even on sophisticated three-dimensional objects. This makes ALDep perfect for applying ultra thin (nanometric) corrosion resistant coatings on many small devices (including jewelery), where thick coatings cannot be used.
In order to get a better understanding of this method, watch the video above.
Sony Vegas Pro 13 Suite was used for making this video – check out their website below:
Sony Creative Software Inc.