Dental Alloy Corrosion: Three Tests, Two Fluids (ISO 10271)

1. Which of the three ISO 10271 tests use artificial saliva?

This is the distinction that decides what you order, and it is the one most often collapsed. ISO 10271 specifies three different corrosion test methods for dental metallic materials, and they do not share a solution.

The static immersion solution is not a saliva at all. At pH 2.3 it is an aggressive acid medium chosen to give a measurable release in a week, and asking for artificial saliva to run it is asking for the wrong fluid. Conversely, running the electrochemical test in the pH 2.3 immersion solution produces polarisation curves that describe no clinical condition.

What we supply is the static immersion solution, made to the clause 4.1.5 composition and dated on despatch, with its measured pH on the certificate. The saliva used for the electrochemical and tarnish methods is a different fluid and is not currently in our catalogue.

2. Why did the current edition of ISO 10271 add a test?

The third edition cancelled and replaced ISO 10271:2011, and its substantive addition was a static immersion test with periodic analysis: the solution is sampled at intervals rather than analysed once at the end.

The stated reason is that the corrosion rate of most dental metallic materials varies over time. A single endpoint measurement gives you a total and hides the shape of the curve, and the shape is where the useful information is. An alloy that releases quickly and then passivates and one that releases steadily can arrive at the same seven-day total while behaving quite differently in a mouth over years.

Practical consequence for ordering: periodic analysis means withdrawing aliquots, and every aliquot removed changes the surface-to-volume ratio unless it is replaced. Decide in advance whether you replace the volume, and plan the fluid accordingly.

3. What is in Fusayama saliva, and which ingredient will not keep?

Fusayama-type saliva carries sodium chloride, potassium chloride, calcium chloride, sodium dihydrogen phosphate and urea. For tarnish work a sulphide is added, and that is where the shelf life comes from.

Sodium sulphide oxidises on standing, losing sulphide to the air. A tarnish test run with a partly oxidised solution understates discolouration, and it does so in the direction that flatters the alloy: the result looks like a pass. This is the single most common way this test gives a wrong answer, and nothing about the solution's appearance warns you.

The practical consequence is that a sulphide-bearing saliva cannot sensibly be bought from stock at all, wherever it comes from: plan the specimens before the fluid, not the reverse. A bottle that waited three weeks for a busy bench no longer does the job it was bought for, and it will not tell you so.

4. How do you read a metal release result on a dental alloy?

Static immersion results are reported per element, normalised to the exposed surface area, and analysed by ICP-MS or ICP-OES. Two points determine whether the number means anything.

The first is the surface area, measured and recorded rather than nominal, because the result is divided by it and a casting is rarely the area its drawing implies. The second is the blank: at the concentrations involved, the metal background of the water and the reagents is not negligible, and for the elements that matter clinically, nickel, cobalt, chromium and beryllium, an uncontrolled background can dominate a low result.

This guide sets out which of the three methods applies and where each goes wrong. It does not reproduce the compositions, durations or acceptance criteria of ISO 10271, which you should obtain from ISO or your national body. Solution parameters cited here are those published in the standard's publicly documented scope.