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Design of protective layers against Corrosion Under Insulation (CUI). The outer protective layer can effectively prevent or block the entry of external moisture and corrosive agents. Outer protective layers fall into two categories: metallic and non-metallic. The selection of an outer protective layer requires taking into account factors such as cost, environmental corrosion, and operating temperature. The most widely used metal protective coatings in industrial applications are aluminizing, aluminum-zinc plating, aluminum cladding, and stainless steel. Under marine climate conditions, the relative humidity of the air is high, and the salt content in the air is relatively large; consequently, corrosion beneath the insulation layer is quite severe. Engineering practice shows that aluminum-zinc coated carbon steel exhibits good corrosion resistance in marine climate conditions. Only when the design of the protective layer structure is reasonable can external moisture be effectively prevented from entering the insulation layer; relevant engineering design manuals can be referred to for this purpose. This article uses the insulation box as an example to illustrate the key points of protective layer design: joints should be overlapped to reduce the likelihood of external moisture entering, sealant should be applied to the seams after overlapping, the upper part of the box lid should be kept inclined to facilitate drainage, and drainage holes should be provided at the bottom of the valve box, among other things. The following points should be considered in engineering design: ① The spacing between pipes should be reasonable, sufficient space for insulation should be provided, and attention should be paid to the distance between pipes as well as between pipes and other facilities; special consideration should be given to the space occupied by the insulation boxes ； ②Reduce the number of openings for insulation on piping equipment; the more openings there are, the greater the likelihood of water and contaminants entering the insulation layer ； ③Equipment attachments should avoid having structures that penetrate the insulation layer or shapes that cause moisture to accumulate ； ④The device should have proper pathways to prevent maintenance personnel from stepping on the insulation cover.