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Corrosion of dental fillings

DENTAL CORROSION

Passive Layers on Metals

We know metals corrode. If you leave a steel rod outside your home, after a period of time exposed to the environment, rust forms. This is one form of corrosion.

This led to innovations of ?passive layers? to protect metals – such as stainless steel which contains a passive film that prevents further surface corrosion.

amalgam

Dental restorations have also evolved down this line. For example, to protect titanium implants from corroding, it usually contains a passive layer made out of titanium dioxide. This gives exceptional corrosion resistance, but even so, it is not completely inert to corrosion attack.

As we know, the mouth is a complex system and under constant bombardment of the ?elements?.

As a result, different types of corrosion can happen in the mouth – all affecting the metals or passive layer on the metal.

We list different forms of corrosion below:

Galvanic corrosion – when a restoration is made out of two or more dissimilar metals (called an alloy), such as in the case of porcelain-fused-to-metal crowns and amalgam fillings, and it comes in contact with oral fluids, there is a difference in the corrosion potential and a flow of electric current results. At a micro-level, a ?micro-battery? is formed – leading to corrosion, albeit at a very slow rate, counted in years. This leads to degradation of the restoration over a long period of time.

Stress and pit corrosion – to the best abilities of the lab technicians, crowns may have small pits and crevices which cannot be polished. More should be stressed of amalgam fillings where it is seldom polished and contains pits and holes after constant use. This pits and crevices concentrate stresses on the restoration and teeth, leading to leakage and fractures.

Microbial corrosion – the passive layer of implants may be disturbed by attachment of microbes on it. The complex interaction between oxygen-needing and oxygen-independent bacteria causes this microbial corrosion. Microbes are capable of forming organic acids that reduces the pH surrounding the implant, which will allow dissolution of the passive layer.

Fluoride corrosion – fluoride ions are very aggressive on the passive layer of implants and this has been widely reported in multiple studies. This is due to the fact that fluoride ions create an acidic environment that is detrimental to this passive layer.