(1) The coating
Black oxidation is a surface treatment where the surface of the steel component is coated with a black iron oxide layer Fe3O4. With black oxidation, we mean hot black oxidation which is significantly different in process and results from cold black colouring and which is also the technically preferred method. The iron in the component itself is used to form the coating. It is therefore not a coating lying on top of the surface, as would be the case with e.g. chromium, nickel, paint, etc., but is actually the result of a direct chemical reaction with the base material. The expert calls this a conversion layer as the substrate conversion means the dimensions and tolerances of the component do not change and the coating cannot be removed or peel off. The coating thickness, which is better known as the oxidation depth, reaches a maximum of 2 µm. With the correct balance between FeO and FeFe2O3, together with the complete oxidation of the surface, perfect black oxidation results in a uniform black colour that offers a particularly high quality appearance after being lightly oiled or sealed with a transparent substance. There are also significant technical advantages to black oxidised surfaces, in addition to appearance, which make their use in mechanical engineering extremely interesting and which are looked at in more detail below.
(2) Production
The steel or iron components to be black oxidised are immersed sequentially in a series of various baths. These are purely chemical processes without any use of external power. First of all, the surfaces are degreased and cleaned, possibly with additional treatment, before being processed in a one to three stage black oxidation bath sequence, finishing off with rinsing and final conservation. As the black oxidation process itself is alkaline, there are no damaging micro-attacks to the surface if the pre-treatment steps are selected appropriately and correct bath management is ensured. The maximum temperature here is below 150 °C and is therefore below standard annealing temperatures. The delivery condition is corrosion protected, either with a dewatering fluid, a preservation oil or an emulsion. The production processes described here are implemented in our company using automatic systems, computer-based program controllers and with the sequence data being recorded. This is carried out to completely exclude any deviations and non-uniformity, such as those occurring in manually operated systems, and to guarantee that the customer receives only high-quality, reliable and economic results.
(3) Workpieces/Components
As workpieces are immersed and masking is not usually implemented, coating formation is continuous without any partial black oxidation. The components can either be passed through as drum goods in a cost-effective bulk goods process or as individually positioned or racked goods. The racked goods are carefully loaded and unloaded by trained personnel – with gloves of course – to avoid any mechanical damage. As an innovative contract coater, we have developed numerous elaborate workpiece carriers for racked goods, ensuring that we can avoid to a great extent or even completely in the majority of cases any visible contact points forming on the contact or hanging surfaces. We can also black oxidise unusually large and heavy components in a special plant. We can rapidly evaluate feasibility for you once we know the dimensions, geometry, weight, material type and surface quality of your workpieces. The knowledge of the precise workpiece geometry (drawing, where possible) allows us to optimise workpiece positioning and correctly take account of any possible scooping or air inclusions. It is important, in all cases, that the components consist of a iron-based material that can be black oxidised, that their surface treatment is the best possible for an optimal result, that they are as clean as possible (with the minimum required preservation agents) and that they are delivered without being treated with silicone oil/grease or wax-based media. Hardened or unhardened steel surfaces that have been freshly turned, milled, ground or honed are particularly suitable for appropriate black oxidation. Workpieces with passive layers, rust or scale must first be treated by pickling; this is yet another service that we offer. As some alloying elements in steel can affect the process, the precise indication of the steel type is extremely useful to determine how this might affect the technical properties resulting from black oxidation. Stainless steels cannot therefore be black oxidised in a classic process due to their high alloy content. However, we can offer other processes, as we do for other metals.
(4) Durability and protective value of black oxidation
Black oxidation treatment is resistant in particular to lubricants such as greases, oils and most oil additives, and is therefore also suitable for numerous technical applications such as component protection against gear oils. The treatment works well for neutral to slightly alkaline environments, but not for acidic environments. The steel is protected against external chemical influences, but effectiveness depends strongly on the exact type and intensity of the chemical attack. The corrosion protection of black oxidation is only achieved in combination with a suitable preservation oil. This corrosion protection meets requirements, e.g. for storage or for indoor use, but is not suitable for outdoor use subject to weather or aggressive environments.
(5) Tribology and technical suitability
It has been discovered in the recent past that black oxidised parts evidence particular technical properties with regards to their relative movements against each other. This effect already applies to individual black oxidised elements, but is quite often far stronger when all components that move together are black oxidised. In general, components designed for sliding or rolling are made of hardened steel and have a very low surface roughness due to machining processes. The most significant technical advantages of black oxidation can be achieved on such components. However, as not all the mentioned advantageous properties may occur simultaneously in all application cases, even on such components, we recommend carrying out a specific test first. With our well-equipped technical centre, we are happy to offer appropriate sample processing so that we can adapt the process parameters during the tests – and for future serial production – to obtain the results required by the customer. The following advantages – confirmed and proven in practice – apply for suitable components:
- The run-in behaviour of the components can be favourably affected in a sustainable manner, creating a basis for subsequent smooth operation.
- Improves the adhesion of numerous lubricants to the surfaces and the subsequent lubrication situation.
- Resistance to harmful environmental influences that occur in operation can be increased.
- The risk of cold welding and various other serious surface damages in steel-to-steel contacts caused by brief breakdowns in the lubricating film can be reduced, increasing operational safety.
- Overall, it can be said that black oxidation certainly increases the service life of components.
These black oxidation properties, which have only recently come to the fore – partly through test results for our customers – mean that they are increasingly being used in technical applications to prolong the service life and operational safety of moving steel components. The comparatively moderate costs of black oxidation and the fact that dimensions are not changed means this process is an ideal supplement and optimisation for an already existing component design.
(6) A special DEWE Brünofix skill
Black oxidation can be implemented using diverse methods and with diverse quality, even if the results initially appear almost identical. The chemicals, concentrations, temperatures, times and precise process control must be precisely matched to the required result. This applies in particular when the above-mentioned technical properties are required in a very narrow tolerance range. A lot of experience and know-how are necessary here to select the right parameters and to optimally ensure the desired effects. We can offer particularly comprehensive knowledge about all aspects of the black oxidation process as we are not just a contract company for black oxidation; we also supply complete black oxidation plants and produce the chemicals ourselves. We have become specialists in this field through decades of intensive cooperation with demanding and well-known customers for technical-optimised black oxidation products. We carry out bath tests rapidly and in detail in our well-equipped in-house laboratory. As developers and manufacturers of the chemicals in question, our engineers are capable of analysing the individual components of the baths and precisely adjusting or modifying them to specific requirements as necessary, instead of – as is often the case – having to redose in one go using complete salt mixtures. This allows us to maintain the bath solution at a constant composition. We set the optimal mix for your workpieces and this, in combination with our modern plant engineering, allows us to offer customers a high and uniform quality level and ensure that the required technical and optical product properties are maintained. Reprints, including excerpts, are prohibited. (CB/KN/TS/07.10.2011)