In addition to external anti-corrosion, some special-purpose square tubes also need internal anti-corrosion treatment.

     (1) The allowable deviation of the thickness of the anti-corrosion layer of square and rectangular pipes and the depth of surface defects such as pits and hollows shall comply with the regulations, and the area of each defect shall not be greater than 5 cm2.

     (2) The sand should be hard, clean, and well-graded natural sand, free of debris, with a content of no more than 2%, and a maximum particle size of no more than 1.2 mm. The gradation should be based on the construction process, pipe diameter, and site construction conditions. 

     (3) The construction of cement mortar lining should generally be carried out after the pipeline is laid, the pressure test is qualified and the soil is compacted according to the design requirements. During lining construction, the pipeline must be in a stable state. If the lining anti-corrosion layer is made first and then the pipe is lowered, the square-rectangular pipe wall is thin and the rigidity is small, and the anti-corrosion layer will be damaged due to large deformation during lifting, transportation, and soil return during construction.

   (4) The inner anti-corrosion layer of cement mortar can be constructed by mechanical spraying, manual plastering, dragging, or centrifugal prefabrication. If it is necessary to use the prefabrication method to make the inner anti-corrosion layer, protective measures should be taken for the anti-corrosion layer during transportation, installation, and backfilling.

     (5) The surface of the cement mortar anti-corrosion layer constructed by the mechanical spraying method is smooth, dense, and uniform in thickness, and the effect is good. When the mechanical spraying method is used for construction, the elbow, tee, square pipe, and the adjacent pipe section of the gate valve can be painted manually, and the smooth gradient section is connected with the mechanical spraying lining.

     (6) Flatness requirements Use a ruler with a length of 300mm to stick to the pipe wall along the longitudinal axis of the pipeline, and the gap between the surface of the anti-corrosion coating and the ruler should be less than 2mm.

     (7) Before the lining construction, the inner wall of the rectangular pipe must be cleaned to remove soft scale, floating rust, soil, grease, welding slag and other attachments; the convexity of the inner wall of the steel pipe shall not be greater than 1/3 of the design thickness of the anti-corrosion layer; The maximum vertical deformation of the pipeline shall not be greater than the design requirements, and shall not be greater than 2% of the inner diameter of the pipeline.

     (8) The water used for mixing must be clean, and there are no substances such as soil, oil, acid, or alkali that affect the strength and durability of cement mortar, and drinking water is generally used.

     (9) No matter what method of construction is adopted, the square tube maintenance process is the key link to ensure that the anti-corrosion layer in the cement mortar does not crack and hollow.