Why is the water quality in the low-pressure waste boiler iron-red?

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Our plant has a low-pressure waste boiler with a pressure of 0.2 MPa. The material used for the shell side is 16MnR, while the tube side is made of 304 stainless steel. The fluids flowing through the tube side are mainly CO2, CO, H2, and steam. The pH value of the water in this boiler remains between 7 and 8. Water is supplied to it using the requirements applicable to high-pressure waste boilers, and the quality of the water has always been satisfactory. However, after shutting down the boiler for half a year and restarting it, the water in the boiler has remained iron-red in color. It is suspected that this is caused by CO2 corrosion due to a leak in the tube side. If inspections reveal no leak in the tube side, then could it be battery corrosion between the different materials (battery corrosion between the tube side and the shell side)? How should this issue be addressed?