Are All Dental Alloys Equal? Evaluating Corrosion Behaviour in Nickel-Chromium Alloys

Published in International Journal of Dentistry — January 2011. Authored by Dr. B. Srinivas Rao and Ramesh Chowdhary.

Metal alloys have been used in dentistry for over a century — in crowns, bridges, partial denture frameworks, and dental implants components. Among the most widely used are nickel-chromium (Ni-Cr) alloys, valued for their strength, castability, and significantly lower cost compared to gold or other noble metal alloys. They are found in millions of dental restorations across the world for full mouth rehabilitation.

But not all Ni-Cr alloys are the same. Different manufacturers produce alloys with varying proportions of nickel, chromium, molybdenum, and other trace elements. These compositional differences — sometimes seemingly minor on paper — can have a significant impact on how the alloy behaves inside the mouth during a smile makeover.

This research by Dr. Srinivas investigated a critical question: do these compositional variations affect how much the alloy corrodes in conditions simulating the oral environment?

Why Corrosion Matters Your mouth is one of the most corrosive environments in the human body. Dental restorations are constantly exposed to saliva (which contains dissolved salts and enzymes), fluctuating temperatures from hot and cold food and beverages, varying pH levels from acidic and alkaline foods, and mechanical forces from chewing and grinding.

When a metal alloy corrodes in this environment, several problems can occur:

* Ion release: Metal ions — particularly nickel — are released into the saliva and surrounding tissues. Nickel is a known allergen, and chronic exposure to released nickel ions can trigger allergic reactions ranging from mild oral irritation to severe contact dermatitis in sensitised individuals. * Restoration degradation: Corrosion weakens the metal structure over time, potentially leading to marginal breakdown, loss of fit, secondary decay underneath the restoration, and eventual restoration failure. * Tissue response: Released metal ions can cause chronic inflammation of the surrounding gum tissue, visible as persistent redness, swelling, or a characteristic dark discolouration of the gum line adjacent to the metal restoration. * Galvanic effects: If different metals are present in the mouth (for example, a Ni-Cr crown on one tooth and an amalgam filling on another), galvanic currents can form between them — similar to a battery — accelerating corrosion and occasionally causing a metallic taste or tingling sensation.

How the Study Was Conducted Dr. Srinivas and his co-author tested three commercially available Ni-Cr dental alloys, each with a different composition. The alloys were cast into standardised specimens and immersed in Fusayama artificial saliva — a laboratory solution specifically formulated to simulate the ionic composition and pH of human saliva.

The specimens were subjected to electrochemical corrosion testing — a precise laboratory method that measures how quickly and aggressively a metal corrodes under controlled conditions. Three key parameters were measured for each alloy:

* Corrosion rate (MPY): how fast the alloy loses material, measured in mils per year. A lower number means greater resistance to corrosion. * Corrosion potential (E corr): the electrical potential at which corrosion begins. A more positive (noble) value indicates greater inherent resistance to corrosion initiation. * Corrosion current density (I corr): the rate of electrochemical reaction at the corroding surface. A lower value indicates slower, less aggressive corrosion.

Key Findings The three alloys showed measurably different corrosion behaviours despite all being classified as Ni-Cr alloys. The alloy with the highest chromium and molybdenum content demonstrated the best corrosion resistance — the lowest corrosion rate, the most noble corrosion potential, and the lowest corrosion current density.

Chromium is the key protective element in these alloys. It forms a thin, invisible oxide layer (passive film) on the alloy surface that acts as a barrier against further corrosion. The higher the chromium content, the more robust this protective film. Molybdenum further enhances this protection by stabilising the passive film, particularly in chloride-containing environments like saliva.

The alloy with the lowest chromium content showed the poorest corrosion resistance — corroding significantly faster and releasing more metal ions into the artificial saliva solution.

What This Means for Patients If you have metal crowns, bridges, or partial denture frameworks in your mouth, the specific alloy used matters far more than most patients realise. A cheaper alloy with lower chromium content may save money initially but could corrode faster, release more metal ions, and fail sooner — potentially costing more in the long run through replacements, allergic reactions, or secondary dental problems.

This is precisely why material selection is not a shortcut at Maxface Dental Clinic. When metal alloys are used in restorations, Dr. Srinivas specifies alloys with proven corrosion resistance and biocompatibility — because a restoration that sits in your mouth for 10, 15, or 20 years must withstand the relentless chemical assault of the oral environment without degrading.

For patients with known nickel sensitivity or those who prefer to avoid metal entirely, Maxface Dental Clinic also offers fully metal-free alternatives including zirconia crowns, PEEK frameworks, and all-ceramic restorations — materials that eliminate corrosion concerns altogether.

About the Publication Journal: International Journal of Dentistry

Published: January 2011

Type: In Vitro Research Study

Read the original publication on ResearchGate.