Typical types and properties of mechanical seal failures

Types of mechanical seal damage failures are: chemical damage, thermal damage, and mechanical damage.

(1) Chemical damage

1) Surface corrosion If the metal surface is exposed to corrosive media, but the metal itself does not corrode, surface corrosion will occur. To prevent surface corrosion, corrosion-resistant materials can be used. The corrosion resistance of stainless steel is better because a protective film of oxide or hydroxide is formed on the surface at this time, so that the metal is passivated and not corroded.

2) Pitting Corrosion Severe corrosion pits produced around the surface of metal materials are called pitting corrosion. There are usually two types of pitting corrosion on the entire surface and local pitting corrosion. When stainless steel is used, pitting corrosion occurs when the passivated chromium oxide protective film is partially destroyed. The way to prevent it is to limit the content of chromium in the metal composition and add nickel and copper. The effects of pitting corrosion are more dangerous than uniform corrosion.

Machinery Seal

3) Intergranular corrosion Intergranular corrosion is a severe corrosion phenomenon that occurs only on the crystal interface of the metal. Although its gravimetric corrosion rate is small, it can corrode deeply into the interior of the metal and can also cause severing damage due to the notch effect. For austenitic stainless steel, intergranular corrosion occurs between 450 and 850 °C, and chromium carbide is precipitated at the grain boundary, which makes the material lose its inertness and cause intergranular corrosion. In order to prevent this corrosion, the material is heat treated at 1050°C, so that the chromium solid is melted and uniformly distributed in the austenite matrix.

4) Stress corrosion Corrosion phenomenon caused by metal materials in a corrosive environment under stress. Materials that are prone to stress corrosion are aluminum alloys, copper alloys, steel and austenitic stainless steel. Generally, stress corrosion occurs under high tensile stress, which first manifests as groove marks and cracks, and then completely breaks.

5) Crevice corrosion Crevice corrosion is a kind of concentration cell corrosion, which occurs in the narrow gap where two parts are close. This corrosion phenomenon is often observed near mechanical seal O-rings. This is because the medium flow is hindered by the narrow gap, and an inert oxide layer is formed. Due to insufficient oxygen supply, the solution in the narrow gap forms a concentration cell with the solution outside the gap, which causes corrosion in the gap part.

6) Battery corrosion This is due to the potential difference between the two metals in the dielectric solution, resulting in battery corrosion and battery corrosion. In order to prevent battery corrosion, structures using two metal combinations with potential difference should be avoided as much as possible. The battery voltage sequence for some metals is shown in the table below:

Electrolyte: natural seawater 3% NaCI solution, temperature is 25 ℃.

7) Cavitation and erosion Compared with the above chemical corrosion and electrochemical corrosion, cavitation and erosion are mostly mechanical factors. Metal materials moving at high speeds tend to be accompanied by corrosion due to cavitation.