1. Introduction

An orthopaedic implant is the most demanding materials application in existence: it must carry cyclic load in a warm, salty, protein-rich electrolyte for decades, shed negligible wear debris, and never corrode. The cobalt–chromium–molybdenum system, standardised in the investment-cast alloy ASTM F75 (Co–28Cr–6Mo) and its wrought relatives ASTM F1537, has served that role since the 1930s — first in dentistry, then in hip and knee arthroplasty. The alloy's defining feature is its chromium content, near 28 wt%, which maintains a passive chromium-oxide surface film whose breakdown would be the failure of the implant.

The engineering references are the standard biomaterials texts, Park and Lakes (2007), together with the microstructural literature of Mani et al. (2011) on deformation-induced phase transformation and of Yamanaka et al. (2013) on nitrogen-enhanced nanostructures.

The system's history explains its chemistry. The cobalt–chromium alloys descend from the Stellite hardfacing family, patented by Elwood Haynes in the early twentieth century, and entered medicine in the 1930s as Vitallium, the cast CoCrMo used for dental and then orthopaedic applications. The 28 wt% chromium level, fixed by that early practice, has survived ninety years of alloy development essentially unchanged — an unusual stability in metallurgy — because it sits at the optimum of the passivity–carbide trade-off: high enough for a bulletproof passive film, low enough that the carbide network can be processed and the matrix retains toughness. Later alloys have added tungsten, nitrogen and processing refinements; none has moved the chromium.

2. The CoCrMo metallurgy

Cast F75 solidifies as a face-centred-cubic cobalt matrix containing roughly 28 wt% chromium, 6 wt% molybdenum and about 0.25 wt% carbon. The carbon combines with chromium and molybdenum to form M23C6 carbides that decorate the interdendritic regions and grain boundaries; the molybdenum hardens the solid solution and supports the passive film; the chromium does two jobs at once — it dissolves in the matrix for corrosion resistance and forms the carbides that give the alloy its wear resistance. The microstructure is therefore a designed composite: a corrosion-resistant, tough matrix carrying a network of hard, wear-resisting carbides.

The wrought grades develop a finer, more uniform structure through hot working, and modern processing — powder metallurgy with hot isostatic pressing, nitrogen alloying — refines the carbide population further. The trade-off embedded in every variant is the same one chromium imposes everywhere: more chromium means better passivity but more and coarser carbides, so the alloy chemist must balance corrosion against wear and workability, all within the tight windows of the medical standards.

The film's behaviour under mechanical disturbance completes the corrosion story. At a bearing surface the passive film is repeatedly damaged and re-formed, and the current transients of re-passivation release metal ions into the surrounding tissue — the microscopic origin of the ion-release figures reported in the implant literature. Chromium dominates this behaviour twice over: it makes the film form faster and it dissolves more slowly than cobalt or molybdenum from the underlying matrix, so a chromium-rich film is both the quickest to heal and the least aggressive in what it releases. The alloy's high chromium content is thus not merely a corrosion reserve but a control on the implant's entire electrochemical conversation with the body — a conversation that runs, quietly, for the life of the patient.

3. The passive film

In physiological saline at body temperature, CoCrMo sits in its passive state: the chromium at the surface is oxidised to a continuous Cr2O3 film a few nanometres thick, which suppresses metal dissolution by orders of magnitude. The film is self-healing — any scratch re-passivates in milliseconds as long as chromium is available at the surface — and it is this property, more than bulk corrosion rate, that qualifies the alloy for implantation. The electrochemical signature is the classic polarisation curve: an active nose at the corrosion potential, then a wide passive region of near-constant, extremely low current density, then transpassive dissolution at high potentials where the film oxidises further and chromium goes into solution.

The film's integrity is a chromium-content matter. In low-chromium cobalt alloys the passive region narrows and pitting becomes possible; the 26–30 wt% window of the medical grades is chosen precisely to keep the film stable across the full range of physiological pH and potential. Impurities that would disturb the film — sulphur, which promotes pitting, and inclusions in general — are restricted to the same low levels the aerospace industry demands.

The wear performance is equally a surface-chemistry matter: the same chromium that builds the carbide particles maintains the passive film between them, so that the bearing surface is simultaneously hard and chemically inert. The two chromium roles reinforce each other under contact — the carbides carry the load, the film prevents the corrosion-assisted wear that would otherwise degrade both — and the alloy's reputation in orthopaedics rests on that cooperation as much as on any single property.

4. Wear and the carbide story

The bearing surfaces of a hip replacement are polished to nanometre roughness and run against polyethylene, ceramic, or historically against another CoCrMo surface. Wear resistance comes from the carbide population: the M23C6 carbides are far harder than the matrix and carry the contact load, while the matrix absorbs deformation. The interplay is subtle — too little carbon gives a soft surface, too much gives brittle networks that shed particles — and the modern alloys control carbon to roughly 0.15–0.35 wt% with processing to match. Wear debris, whatever its chemistry, is the implant's political and biological problem, and the industry's response — better surface finishes, harder films, alternative couples — has always circled back to the same chromium-built material base.

Between melting and implantation lies a quality chain that dwarfs the metalworking itself. Every ingot is analysed against the ISO window; every subsequent step — casting trees, HIP, machining, polishing, cleaning, passivation, packaging — is documented and traceable to that analysis; and the finished device carries the history of its metal into the operating theatre. The chromium metal's certificate sits at the head of that chain, and the medical industry reads it with aerospace strictness plus one further demand: the elements that matter biologically — the heavy metals and the gases — are watched even more closely than the structural residuals, because the implant's interface with living tissue is an electrochemical and biological system, not merely a mechanical one. Chromium, the element that builds the interface, is therefore the most carefully specified element in the alloy.

5. Processing: from charge to implant

Tribocorrosion completes the picture of what the implant surface endures. At a bearing articulation, the passive film is repeatedly scraped away by contact and re-formed by the alloy beneath; the re-passivation consumes chromium at the surface, and the alloy's chromium reservoir — 28 wt% distributed through the matrix — is what sustains the cycle indefinitely. When the couple involves polyethylene, the metallic surface still sees protein-containing fluid and occasional third-body particles; when it is metal-on-metal, the two CoCrMo surfaces polish each other while the same film heals both. In every case the engineering statement is the same: the implant survives because chromium re-passivates faster than the environment can attack, and the design reserves of the system are measured in chromium concentration.

The route matters as much as the chemistry. Investment casting produces the complex, near-net shapes of hip stems and knee components; the cast structure is then homogenised and hot-isostatically pressed to close microporosity; machined and polished to the required finish; and cleaned, passivated and packaged under medical-grade control. Wrought F1537 grades serve where higher fatigue strength is needed, produced through melting, hot working and controlled recrystallisation. At the head of every route sits the charge: virgin cobalt, molybdenum, and chromium metal — and the chromium carries the usual sixteen-element discipline, because an impurity that reaches the implant surface rides in the passive film for the next thirty years.

The regulatory frame is worth stating plainly. Implant alloys are governed by the ISO 5832 series, which fixes the composition windows — for CoCrMo castings, 26.5–30 wt% Cr, 4.5–7 wt% Mo, carbon to 0.35 wt% with the tramp and gas limits — and by the device regulators' requirement that every production lot be traceable to its melt certificates. Biocompatibility is assessed through cytotoxicity, sensitisation and implantation testing, but the chemical foundation of all of it is the passive chromium oxide film: the tests pass because the film holds, and the film holds because the chromium content and cleanliness of the alloy hold. The implant industry therefore consumes chromium under a double discipline — metallurgical and regulatory — and its certificates read like the aerospace industry's with a patient at the end of the chain.

6. What the medical industry buys

Additive manufacturing has added a new processing chapter without changing the chemistry. Laser powder-bed fusion builds CoCrMo implants layer by layer from gas-atomised powder of the same 28Cr–6Mo composition; the rapid solidification produces an extremely fine cellular structure that can exceed the properties of cast material after heat treatment, and the technology is now routine for custom and porous-surface implants. The powder route raises the purity discipline one notch further, because every gas pocket or impurity particle in the feedstock prints itself into the component: medical-grade CoCrMo powder is therefore specified with oxygen and nitrogen in the low hundreds of ppm and full traceability to the chromium and cobalt certificates behind it.

Medical-device makers buy chromium in the same forms as the aerospace melt shops — refined block and low-gas powder — but their certificates run tighter in the elements that matter biologically and electrochemically: sulphur for pitting, lead and the tramp metals for toxicity, gases for cleanliness. The regulatory layer adds traceability: every heat of alloy is tied to its charge certificates, and the implant's paper trail begins with the chromium metal's own analysis. In a very concrete sense, the safety of the implant is specified at the chromium supplier's desk, months before the alloy is melted.

What lies ahead continues to circle the same chemistry. Additive manufacturing is moving from custom implants toward series production, with the powder specification becoming the implant specification; surface engineering — ceramic coatings, ion implantation, biomolecule grafting — modifies the outer nanometres but rests on the same chromium-built passive foundation; and the demographic growth of arthroplasty keeps expanding the installed base. The CoCrMo system's next ninety years will look different in process and regulation, but the load-bearing surface will remain, as it has been since Vitallium, chromium oxide over a chromium-rich matrix.

The final word belongs to the interface. Everything this article has described — the castings, the carbides, the certifications — exists to maintain a film a few nanometres thick, and that film exists because the alloy carries chromium at 28 wt%. The implant industry's sophistication, measured in machines and regulations, rests on that elementary chemical fact; and the chromium industry's contribution to human mobility, measured over a century and millions of patients, has been to deliver the element that makes the film possible. Few applications of chromium are less visible, and few matter more to the people they serve.

7. Outlook

The demographic forces behind arthroplasty are one-directional, and the CoCrMo system, after ninety years, remains the reference material for load-bearing implants. Its evolution — nitrogen alloying, additive manufacturing, graded carbides — changes the microstructure but not the chemistry: chromium near 28 wt%, maintaining a nanometre-thin oxide film that stands between the metal and the body. As long as artificial joints are made of cobalt, they will be protected by chromium.