Ionic “armor” for steel: how Russian physicists protected an aircraft alloy from corrosion

A new era of anti-corrosion protection

A group of Russian scientists — specialists from the Center for Advanced Methods of Mesophysics and Nanotechnologies at MIPT, the Institute of High Current Electronics of the SB RAS, and Tomsk State University — have developed a way to significantly increase the corrosion resistance of aviation steel grade VNS-5 (13Kh15N4AM3) without degrading its mechanical properties.

The technology is based on high-dose ion implantation — bombarding the surface with argon and chromium ion beams. As a result, a strong protective layer up to 100 nm thick is formed that resists seawater and biofouling.

Senior researcher at the Laboratory of Topological Quantum Phenomena in Superconducting Systems at MIPT, Viktor Semin, noted that Russian physicists achieved record long-term corrosion resistance for ion implantation methods. Scientists expect this development to change approaches to metal protection in aerospace and marine industries and become a basis for new grades of high-strength steel that effectively withstand aggressive environments.

The problem: how expensive steel fails under seawater attack

Steel 13Kh15N4AM3 belongs to heat-resistant high-alloy austenitic–martensitic alloys. It has high toughness, ductility, and resistance to stress concentrators. Its composition includes roughly 15% chromium (Cr), 5% nickel (Ni), and 3% molybdenum (Mo). The material is widely used in aviation and space technology, in critical assemblies of aircraft and ships.

However, tests of VNS-5 steel samples under real conditions in Vietnam revealed an unpleasant feature: even such an expensive and strong alloy quickly deteriorates under seawater, microorganisms, fungi, and moss. For aviation and shipbuilding this is critical, because such alloys are used in bolts, anchors, and other structures on which safety depends.

Technology: ion beams against degradation

Ion implantation showed impressive results: corrosion current decreased by a factor of 7.8 compared with untreated samples. For comparison, chemical passivation provided only a minor effect.

The difference lies in the structure of the protective layer:

• with ion implantation, a dense chromium oxide (Cr₂O₃) film tens of nanometers thick is formed that resists aggressive environments;
• with chemical passivation, a less durable layer of chromium hydroxide and iron oxides forms, which does not provide long-term protection.

The main advantage of ion implantation is the absence of an interface between the modified layer and the substrate, which eliminates the delamination risk typical of conventional coatings.

Materials, methods, and results

• Chemical passivation: immersing samples in a solution of 10% hypophosphorous acid (H₂PO₂) and 0.05 M potassium dichromate for 140 h.
• Ion implantation: preliminary grinding with SiC abrasive paper (Ra = 0.16 µm), then treatment with Cr⁺ and Ar⁺ ion beams formed in a unique magnetron sputtering system with electron injection (RF patent; no world analogues).

Results:

• ion-treated samples showed the highest charge-transfer resistance and maximum polarization resistance;
• corrosion rate decreased by a factor of 7.8 compared with the original steel;
• electroactive layer thickness after ion implantation was ~13.7 nm versus ~8.9 nm after chemical passivation;
• XPS analysis showed that the protective layer is dominated by Fe₂O₃, Fe₃O₄, Cr₂O₃ oxides and a small amount of CrO(OH) hydroxide providing a passivating effect.

Why it matters

Corrosion is one of the main causes of premature failure of metal structures, especially in marine and coastal environments. Estimates put annual economic losses in developed countries at 3–4% of GDP. Each year 1–1.5% of all industrial metal is lost, and about 20% of annual production is lost irreversibly. Therefore improving material durability is not only a scientific task, but an important contribution to economic efficiency and engineering safety.

What makes ion “armor” good:

• Durability — the layer lasts as long as the part itself.
• No seams or interfaces — no risk that protection will peel off.
• Minimal thickness — does not change part dimensions or weight.

Chromium oxide formed during ion implantation is a key component of stainless steel. It forms a self-healing barrier resistant to moisture and aggressive environments.

Industrial solutions: what you can already buy

If you work with corrosive environments, consider cold-rolled strip of corrosion-resistant steels available at PZPS. The range includes steel grades:

• 20Kh13 — a martensitic corrosion-resistant steel with ~13% Cr. It has moderate resistance in mildly corrosive environments, but is inferior to austenitic steels in seawater.
• 12Kh18N9 — an austenitic steel resistant to corrosion in atmospheric conditions and mildly aggressive environments; good in fresh water, but needs protection in seawater.
• 12Kh18N10T — an improved variant of 12Kh18N9 with titanium added, increasing resistance to intergranular corrosion.
• 10Kh17N13M3T — a high-alloy austenitic steel with molybdenum, resistant to chloride corrosion and seawater.

Precision alloys are also available:

• 40KKhNM, 36NKhTYu — have high elasticity, but corrosion resistance depends on the environment; they need additional protection in marine atmospheres.
• Kh15Yu5, Kh23Yu5, Kh23Yu5T, Kh15N60-N, Kh20N80-N — alloys with high electrical resistivity; their corrosion resistance is usually lower than that of austenitic stainless steels and depends on service conditions.

Ion implantation has proven effective both in laboratory tests and in real service. Scientists plan to expand research and create new generations of steels capable of withstanding the most aggressive conditions.

“Based on this work we plan to submit an RSF project titled ‘Development of a set of electrochemical and ion-beam methods for surface modification of austenitic–martensitic steels VNS-5 and VNS-74 to improve their mechanical properties and service life in marine environments’ and broaden the research scope,” said Viktor Semin.