The browning of the trunk in and of itself is not a problem but rather something that all cacti tend to do as they get older. they develop a corky, barky type base. However on the left side of the plant from what I can see in the photo are small round things. This looks like scale, a sucking insect. It may have added to the barkiness of the plant. They are difficult to get rid of but try a mix of 50/50 water and rubbing alcohol and a few drops of dish washing soap in a hand held spray bottle. Spray the affected are well and use a soft brush or sponge to gently rub down the area. This should remove the existing scale. You may need to repeat every now and then if they show up. Spray the affected area down with regular water after treating.
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How can you tell if it is corking or a scale? How can you tell if the scale is still alive?