PASSIVATION OF NON-STAINLESS STEELS

The passivation of non-stainless steels encompasses a set of surface and thermal treatments aimed at increasing their corrosion resistance and, in some cases, also improving mechanical or aesthetic properties. Unlike stainless steels, which naturally generate a protective layer due to their high chromium content, carbon steels and common alloys require specific interventions to achieve a similar level of protection.

These processes do not transform the steel into stainless, but they do reduce its reactivity to oxygen, moisture, and other chemical agents, thereby increasing its service life and expanding its field of application.

Bluing, also known as black oxidation, is based on inducing the formation of a thin layer of magnetite on the steel surface by immersing it in hot alkaline solutions that cause a controlled chemical reaction. This dark, uniform finish not only provides an aesthetic value but also reduces light reflection and offers moderate protection against oxidation. In practice, it is often complemented with oils or waxes that reinforce the impermeability of the formed layer, thus prolonging its effectiveness in humid conditions or intermittent contact with water. 

Phosphating is a chemical process that coats steel with a microcrystalline layer of metal phosphates, generated by reaction with acidic solutions containing phosphate compounds. This film, with a matte appearance and porous texture, acts as an excellent base for paints, lubricants, or oils, while providing a first defense against corrosion. Its use is widespread in the automotive industry, the manufacture of fasteners, and the protection of mechanical parts subjected to friction or humid conditions. 

Nitriding introduces nitrogen atoms into the steel surface, forming extremely hard and stable compounds known as nitrides. It is carried out in controlled atmospheres using methods such as gaseous nitriding with ammonia, plasma nitriding (ion nitriding), or liquid nitriding with molten salts. The resulting surface exhibits significantly superior hardness, improved wear resistance, and chemical protection against corrosion, making it particularly valuable for parts subjected to intense stresses or aggressive environments. Furthermore, by not requiring excessively high temperatures, nitriding does not deform the parts, maintaining their dimensional precision. 

Burnishing is a mechanical treatment that compacts the steel surface through controlled friction, closing micropores and smoothing roughness. This surface work hinders the penetration of moisture and contaminants, delaying the onset of corrosion. While it does not generate a chemical protective layer, its physical effect effectively complements other passivating treatments. 

Chemical blackening shares with bluing the purpose of darkening and protecting the surface, but it is carried out at lower temperatures and with specific reagents that accelerate the formation of protective oxides. This allows for more precise control of the finish and rapid execution, which is particularly advantageous for small or decorative parts. 

Nickel plating is a coating deposited with nickel that acts as a continuous barrier against corrosion and improves the surface finish of steel. It can be performed electrolytically, using electric current to co-deposit nickel onto the part, or autocatalytically (electroless nickel plating), where the bath itself reduces nickel ions without the need for electricity. Electroless nickel plating stands out for its excellent uniformity of thickness on complex geometries and its amorphous or nanocrystalline microstructure when it incorporates phosphorus, which increases corrosion resistance and reduces porosity. For anti-corrosive applications, sufficient thicknesses are sought to ensure layer continuity; the range varies depending on exposure, but the practical criterion is to ensure there are no open pores or discontinuities on edges or recesses. In steels with high hardness or those susceptible to hydrogen embrittlement, electrolytic processes require subsequent dehydrogenation through a mild baking to eliminate absorbed hydrogen and preserve toughness.

Nickel plating can be combined with sealants or top coats to enhance its performance. A classic example is the copper–nickel–chromium system in decorative finishes, where nickel provides the primary protection, while adjacent layers improve adhesion and appearance. In moderate chemical environments, high-phosphorus electroless nickel offers outstanding protection due to its low film conductivity and reduced tendency to micro-cracking. For parts subjected to wear, certain nickel platings can incorporate fine particles in co-deposit (e.g., PTFE or silicon carbide) to balance friction resistance and durability, maintaining the barrier function without compromising the integrity of the base steel.

Common galvanizing is a process that protects steel by applying a zinc coating that shields it through a double mechanism: physical barrier and cathodic sacrificial protection. When the system is locally damaged, the zinc oxidizes preferentially, maintaining the steel's potential in a safe zone and delaying the onset of rust. Hot-dip galvanizing submerges the part in molten zinc and develops Fe–Zn intermetallic layers on which metallic zinc solidifies, achieving high thicknesses and remarkable durability in severe outdoor environments. Electrogalvanizing, in contrast, deposits zinc electrolytically at a lower thickness with great uniformity and good appearance, making it ideal for precision components or interiors with controlled exposure. After zinc plating, a conversion passivation is usually applied, mainly based on trivalent systems today, which seals the microstructure, improves white rust resistance, and provides specific tones if required.

In high-strength steels, the electrolytic process can introduce hydrogen, with the consequent risk of embrittlement; therefore, dehydrogenation baking shortly after coating is a recommended practice. The selection between hot-dip galvanizing and electrogalvanizing depends on the severity of the environment, dimensional tolerances, and aesthetic requirements. In exposed structures, hot-dip galvanizing offers a superior service life due to its greater sacrificial reserve and the robustness of the intermetallic layer, while for fine fasteners, housings, and tight assemblies, electrogalvanizing facilitates interference-free assembly and allows transparent or colored passivations. When the design combines different materials, the resulting galvanic cell should be considered: zinc reduces the risk of steel corrosion, but its proximity to more noble metals can accelerate coating consumption if a persistent electrolyte exists.

Chrome plating encompasses both decorative chrome plating and hard chrome plating, with different purposes and layer architectures. In decorative chrome plating, the chrome is very thin, and its main function is to provide shine, color, and surface stain resistance; the actual corrosion protection is provided by the underlying nickel layer, often structured in multiple sub-layers to hinder the propagation of pores. This system requires meticulous steel preparation, with degreasing, activation, and, in many cases, a copper layer that regularizes the surface and improves nickel adhesion. The result is a stable finish with good gloss retention, suitable for components exposed to moderate atmospheric agents and high aesthetic demands.

Hard chrome plating deposits much thicker layers with the aim of increasing wear resistance, surface hardness, and dimensional stability in service. It is common to find controlled micro-cracks in the deposit, which help retain lubricants and dissipate stresses, although this micro-cracking necessitates corrosion evaluation if the part operates in wet or saline environments. In critical applications, electroless nickel pre-layers or subsequent sealants are combined to improve corrosion behavior without sacrificing the tribological function of the chrome. Given that traditional chrome plating processes have been associated with hexavalent compounds, many industrial lines have migrated to trivalent chrome technologies and alternate layer architectures that maintain performance while improving environmental and regulatory compatibility. In all cases, the success of chrome plating on non-stainless steels depends on prior preparation, thickness control, and stress management to prevent delamination or premature failure.