How to pay attention to metal corrosion under the insulation layer?

Hot-dip galvanizing, foaming, application of high-temperature resistant paint, replacement with stainless steel.

We know that most insulation is rock wool or ultra-fine glass wool. These materials are made by calcining and bonding minerals. Since minerals contain large amounts of inorganic salts, the insulation materials produced after treatment (or sometimes without treatment) still contain harmful components such as chlorides, fluorides, and sulfides. When these insulation materials come into contact with metal surfaces, corrosion occurs as a result of this contact. The reason for this is that the insulation materials contain water-absorbing substances such as K2O and Na2O, which undergo chemical reactions with the moisture in the air, absorbing large amounts of water. Since these materials are composed of columnar fibers, a siphoning effect easily occurs, causing the insulation materials to become wet. The presence of soluble salts in these materials further enhances the absorption of water by them. The adsorption and condensation of water make the insulation material damp, forming a water film on the metal surface. The dissolution of soluble substances turns the water film into a strong electrolyte solution, facilitating the electrochemical corrosion of metals. Analysis shows that the chloride ions in the glass wool in close contact with the metal surface are as high as 1.6%. Due to varying degrees of coverage by insulation materials, or because the waterproof layer loses its effectiveness, it becomes difficult to ensure an adequate supply of oxygen, resulting in oxygen-deficient areas. In the damaged areas (those with severe corrosion), there is an adequate supply of oxygen; this results in the formation of an oxygen concentration cell between two adjacent areas where there is a continuous layer of electrolyte, thereby causing electrochemical corrosion of the metal. It is recommended to address this by paying attention to the waterproof structure of the insulation layer and ensuring proper waterproofing. To provide corrosion protection for metal surfaces with insulation layers, coating can be applied using enhanced corrosion prevention methods.

If such pipes are operated at 200°C for an extended period, they shouldn’t corrode to such an extent. It seems that there are large fluctuations in temperature; moreover, surface condensation and external water exposure at low temperatures can cause such severe corrosion.

If it is a start/stop device, there will be no problems when it is in operation. However, when operations are suspended and the insulation layer lacks proper waterproofing, inadequate corrosion protection of the metal surface can lead to severe corrosion.

There must be water getting in through the insulation sheeting on the outside!

Is the operating temperature always 200 degrees? If insulation is in place, corrosion should reach its limit at around 180 degrees... So what some people have said is correct – there should be significant fluctuations in temperature, and the insulation essentially becomes ineffective (it’s highly likely that either the material used for insulation was chosen incorrectly or that the quality of that insulation material is poor). The solution is to use appropriate paints and insulation materials. -------------------------------------------------------------- In my workplace, it takes 1 year for such corrosion to become apparent

One of the typical conditions of corrosion beneath the insulation layer is caused by crevice corrosion at the flange joints.