In this article you will learn what a hard chrome plating is, how it is made, what is its structure and which are its advantages.
Hard chrome plating is a technological process that ensures a product has increased wear resistance, reduce friction, and most important, gives increase corrosion resistance to a metallic product aspect critically important especially in the hydraulics industry. Chrome plating is an ever changing or transforming process, one that has brought forth many improvements over the past few years, but also a lot of marketing “attacks” (example thousands of hours of Rating 10) – this will be something I will write about in a future article.
How is hard chrome plating achieved?
Hard chrome plating is an electrolytic process utilizing a chromic acid based electrolyte, or easier still, coating of metallic chrome that is deposited electrolytic on a metallic material.
The process is quite simple and requires energy, chromic acid, big enough baths or lines (if working in continuous chroming) depending on size/lengths of chromed products and anodes. In the chroming process, the product that is going to be chromed, becomes a “cathode” that gets through an “anode” that is found in the chroming bath. Thus, the chromium metal gets build on the metal surface that we want to chrome. A typical chroming process is made out of 4 different stages:
- activation baths (anodic attack)
- chroming cell (baths)
The chroming process depends on many factors:
- the surface quality of the metal that is going to be chromed (roughness, raw material defects such as holes, overlapping of material…),
- damages from product manipulation such as dents or hits,
- parameters that are being used (current, chroming solution, catalysts, temperature) during the chroming process. Any variation within any of these parameters can cause small or big problems on the chromed batches such as porosity, exfoliation of the chrome layer, surface color difference of the chrome layer and many others.
The micro-structure of the chrome coating is influenced by how you managed to control the hydrogen evolution and the evolution of the chrome film being deposited on the cathode. Basically the micro-structure of the chrome coating is ranging from tight coatings with inner tension to tension free coating with up to 2000 micro-cracks per cm. The chrome is deposited in a hexagonal crystal, reorganizing itself into a tighter centered cubic structure, setting free the buildup Hydrogen. The pillar structure of conventional coatings implies hardness and crispness. The classic hard chrome coating is micro-cracked and tension-free but not tight. Chemistry controls the release of inner tensions and transforms them into micro-cracks measured as number of cracks per cm – as results we may conclude that hard chrome is inherently micro-cracked.
The structure of the chrome film is also affected by exactly the byproduct of the chemical reaction during chroming – the release of the hydrogen. Basically at a critical point, the hydrogen buildup will be released eventually and this will create a fine “exit” point on the surface of the chrome product, point that is prone to corrosion. In order to prevent such an issue, at regular intervals the hydrogen buildup is intentionally made to release itself by flipping the bar and by adding a circular movement on the bar.
Advantages of hard chrome plating
- Corrosion resistance
Corrosion resistance is given by the chrome oxide that is being formed as a result of the chromic acid oxidizing the outmost surface layer. Basically the chrome oxide creates a fine passive film that acts as sealant on the total chromed surface – this oxide acts as “fillings” that “corks” all surface imperfections, pitting and micro-cracks, thus ensuring corrosion resistance. This oxide film is extremely thin up to 5nm and is vulnerable to certain types of wear, although the film is restoring itself in oxidizing environment. In order to potentiate the lifespan of this film, various treatments of the chrome surface are being carried out, such as chemical passivation or polymers.
- Wear resistance
Hard chrome is resistant to abrasive wear (moving particles plans away the surface coating) as long as the surface pressure of the abrasive particles are low. Limited space and hard particles can peel off chromium from the surface accelerating destruction of hard chrome coatings.
Hard particles hitting the coating with high speed in a path perpendicular to the surface can lead to erosive wear and destroy the chrome oxide film initiating corrosion attack provided that the flowing media is acidic and reducing.
- Friction resistance (roughness)
The chrome oxide layer provides low friction between hard chrome and other metals. However friction is also influenced by the roughness of the chromed raw material as hard chrome does not possess any leveling / filling ability at all, chrome just copies the aspect of the chromed surface.
- Surface hardness
At around 1000 HV0.1, chrome plating is harder than most industrial abrasives and steel counter faces. Unusually, it combines this hardness with a degree of toughness, so the deposit can stand up to high stress contact.
My next article will be released soon and will be related to the Corrosion Resistance.
If you want to learn how to achieve it, how to control it and why it is so important in the hydraulic business, please don’t miss my next post.