Oil-based coatings can be used to enhance the corrosion resistance of metals and therefore increase their lifetime. Such oils can be applied either by spraying or by using a brush on metal substrates in their real application environments. Recently we performed an accelerated corrosion test on different oil-based coating products for our client Sia Auto Truck Studio, who were particularly interested in the performance of a spray-type oil based coating – an NHOU Rust Prevention Spray. As the name suggests, the coating can be sprayed on the substrate directly from the bottle, which creates an uniform coating. This is achieved by using a proper combination of oil viscosity and spray bottle design, which creates an expanding cone of expelled oil particles, that create a defect free coating on the metal substrate. The corrosion test was performed on 90×90 mm low carbon steel plates, that are extremely vulnerable to corrosion. For testing, the Machu test was used, which is essentially an accelerated corrosion test in an acidified salt solution with hydrogen peroxide. This test allows to obtain results already within 48 hours, which is useful for quickly gaining feedback on the performance of materials and coatings before carrying out long-term immersion or salt spray tests. As can be seen from the photo in Figure 1, the coated sample remained relatively unharmed after the test, exhibiting only a few individual sites of corrosion in the central area. Only the edge of the sample suffered damage from corrosion as it was problematic to coat for this particular test. Overall, the coating exhibited good performance and should therefore be tested further with long-term immersion and salt spray tests, electrochemical techniques and finally in real application environment.
A large portion of the annual cost of corrosion can be attributed to the corrosion of pipes and ventilation systems. However, the corroding surface is mostly inaccessible in these conditions and therefore the detection and monitoring of corrosion for evaluating the need for possible maintenance is problematic.
Our solution to the problem is a hand-held corrosion monitoring device, that is capable of detecting the corrosion that takes place inside pipes while the measurements are done outside. We currently have a working prototype but we want to investigate also alternative approaches in order to come to the market with a user friendly product.
The development of the prototypes is funded by Captain Corrosion OÜ and Prototron.
Need to test the corrosion resistance of your material or a protective coating?
In collaboration with the University of Tartu we can do both chemical and electrochemical tests to simulate real or even extreme conditions in order to evaluate the performance of your sample.
Contact us if you are interested in corrosion testing!
Chemical tests – Studied substrates are exposed to a corrosive environment similar to the real conditions where it will be used later on. We can also alter the conditions of the environment to make it more corrosive by adjusting the pH and temperature or include UV light. A common example would be a test of series to compare the quality of stainless steel samples obtained from different suppliers. Another example would be the evaluation of different protective coatings on metal substrates.
Electrochemical tests – Corrosion can electrochemically be accelerated and this allows to quickly obtain reliable information about a materials or protective coatings corrosion resistance. For instance, certain metal alloys can be immersed in a salty water for years before it corrodes while electrochemically we can evaluate its long-term performance within a hour.
Microscopy – In addition to corrosion tests we also do microscopy studies of the tested substrates in order to get additional information about the type of corrosion. For example, Pitting corrosion often occurs undetected as it stats as a tiny hole on the surface and forms a network of tunnels inside the substrate, thus greatly degrading its mechanical properties. In contrast, uniform corrosion initially affects the aesthetic appearance of a material and mechanical properties are not much affected if the problem is dealt with.
One of the most widely used methods to counter the degradation of a materials mechanical properties to due to corrosion is to use more material. If the speed of uniform corrosion is known (mm per year), then it is possible to calculate the required thickness of the material, so that it can serve its purpose during its expected lifetime. The Steel Bridge in Portland (USA) is just one example where this method is used. Protective coatings are also often applied in order to slow down the corrosion of the construction material even more.
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.
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!
Anodizing aluminum is the ultimate technique for enhancing the corrosion resistance of automobile and aircraft parts, creating nanoporous templates for nanotechnological applications and making scratch resistant casings for electronic devices (cell phones, laptops etc). Anodizing is basically a 3 step process, that consists of a pretreatment for preparing the metal surface, anodizing for creating the anodic oxide layer and sealing the pores for enhancing the corrosion resistance. In the following steps youll need to use protection like gloves and goggles in order to keep yourself safe and also to prevent the contamination of the metal that will be anodized. Anodizing should also be performed in a very well ventilated room – the acidic vapours will cause severe damage to lungs during a prolonged exposure. These vapours also quickly corrode vulnerable parts in nearby electronic systems – so dont keep any valuable devices in the anodizing room.
Before starting with the pretreatment i recommend attaching the aluminum piece on a holder (aluminum wire for example) made out of the same aluminum alloy – that way youll prevent the contamination of the metal piece in the next steps as you no longer need to touch it. This holder will later also serve as the electrical connection in the anodizing process.
In order to remove the organic contamination, soap and water can be used for cleaning away most of the dirt. After rinsing the aluminum piece with deionized water, the final cleaning needs to be done with acetone. Before anodizing however, we need to remove the natural aluminum oxide layer from the metal surface. For that purpose the aluminum piece is dipped into a sodium hydroxide solution for a short time. The bubbling of hydrogen indicates that the oxide layer has been removed and that sodium hydroxide is reacting with aluminum. After removing the natural oxide layer the aluminum piece is rinsed with distilled water. If you have freshly polished the aluminum substrate, then you can likely skip the chemical cleaning and can go straight to anodizing – that saves you a lot of money. Just be sure you dont contaminate the freshly polished surface.
For anodizing a two-electrode setup is used, where the anodizable aluminum piece is the anode. The aluminum object is connected to the positive lead (usually red). The negative lead is connected to the cathode which can be made of stainless steel. Both electrodes need to be seperated and parallel to each other. However, in order to get a uniform oxide layer all over the anodized plate, i recommend using a stainless steel bath as a cathode instead. The bath should match the size and shape of the anodizable substrates.
The anodizable substrate needs to be completely immersed into the electrolyte before starting the process. The porosity and thickness of this oxide layer depends on the electrical parameters, type of electrolyte, its temperature and anodizing time. For example hard – scratch resistant oxide layers are done with type III anodizing in sulphuric acid, at near freezing temperatures and with lower current densities.
In our experiment we used a 10% sulphuric acid solution, a current density of 2 A / dm2 and the anodizing time was 30 minutes. Since the total surface area of the substrate was 3 dm2, the anodizing current was set to 6 ampers. The temperature of the solution was around 22C at start but it had significantly increased when we measured it again after anodizing. So if there is a need to use higher current densities for anodizing, i recommend using a cooling bath around the anodizing bath. In industrial processes a constant anodizing temperature needs to be ensured.
During the anodizing process a nanoporous oxide layer is generated at the cost of aluminum and this alters the appearance of the aluminum piece. These pores are so small, that they are only visible with a powerful scanning electron microscope. For making corrosion resistant coatings, these pores need to be completely sealed and there are several ways to do it. One such method is hydrothermal sealing, which is basically keeping the anodized substrate in boiling water. As a result aluminum oxide is partially turning into aluminum hydroxide, which takes up more space and seals the small pores. Another popular method is dipping the freshly anodized substrate into paint, which is immediatelly sucked into the pores. This significantly increases the metals corrosion resistance and also gives it an awesome appearance.
The nanoporous aluminum oxide can also be used as a template in nanotechnologial applications. In our case we electrochemically deposited silver into these pores and then removed the aluminum oxide with sodium hydroxide. As a result we got silver nanowires with well defined length and diameter.
Anodizing is quite easy and with some practice, its a powerful technique for treating aluminum objects for personal or commercial purposes.
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.
Anodizing is an electrochemical process where a thicker oxide layer is grown on the material. This is useful for improving an objects corrosion and wear resistance, manufacture nanoporous templates or give the material a decorative appearance. Not all materials can be anodized however as their oxides are not dense and hard but the technique has been widely used on aluminum, titanium, zinc, magnesium and their alloys.
The anodizing system consists of a power source, anodizing bath, electrolyte and anodizable material. The bath is usually made from a chemically resistant conductive material such as stainless steel and serves as a cathode. The anodizable material serves as an anode and is placed inside the anodizing bath with the electrolyte. Both the anode and the cathode need to be connected to the power source. The grown oxide layers properties depend on the material, used electrolyte, temperature and electrical parameters used for anodizing.
In order to produce a uniform oxide layer, the substrates are also treated before the process. The main problem is usually organic contamination of the surface, which prevents growth of the oxide layer. This is removed with organic solvents such as acetone. Often the thin native oxide layer is also removed via etching before the anodizing.