Corrosion of valve flange bolts
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As shown in the image, the bolts on the flange of the oxygen control valves are corroded to half, while the side flanges are not corroded. This is the case for several oxygen control valves – what could be the cause?
Is it caused by acid rain? There is water accumulation on the flat surface; rainwater doesn’t stay on the sides, so it dries quickly.
It’s probably caused by bolt tightening, which damaged the surface coating It’s not visible to the naked eye, but it will rust over time.
Water stains can accumulate on the bolts and gaskets located on the flange surface, creating an electrochemical corrosion environment that causes the bolts to oxidize and rust
Liquid is dripping from above, and water accumulation at the upper bolt area has caused rusting； The vertical surface on the side does not allow water to accumulate, so the bolts have not rusted.
Oxygen pipelines are usually made of stainless steel (304), and the bolts on such pipelines are also typically made of stainless steel. Therefore, the bolts and nuts on the side of the flanges that you see should be made of stainless steel, which is why they do not corrode. The material of the nuts and washers on the valve cover of the control valve is likely to be incorrect; it should be chromium-molybdenum steel or 35# or 45# steel, as these materials are more prone to electrochemical corrosion and crevice corrosion under the effect of rainwater. It is recommended that replacement be carried out during major overhauls. The latest version of SH/T3501 requires that, at the time of acceptance, alloy steel valves must undergo spectral testing on the valve body, valve cover, and valve cover bolts, in order to ensure that the material of all components of the valve meets the specified requirements.
Take a look at what type of steel stamp is marked on each nut
More or less. I think the bolts on top aren’t made of stainless steel either; they look different from the stainless steel ones on the sides