I have some questions if you don't mind:). Did both your cacti start getting this at the same time? How much light do they get? Is the brown color around the whole circumference of the plant? You may have a bug like spider mite:(

Thanks for your response! :) The first one started getting brown about 2 weeks before and the second one just started doing this. The first one has the brown colour around the entire plant, while the second one is kind of randomly distributed around the base. They get around 3-4 hours of sunlight through the window from about 6:00 am- 9/10 am until the sun starts facing in a new direction and then I bring in the grow light for about 12+ more hours! I also repotted the first plant 3 days ago and didn't notice any mites, but maybe I should check the second to be sure :/ I hope it's not spider mites, I just had to take out mealybugs on most of my cacti :(

Corking starts at the bottom and slowly continues upward; if it started in the middle or other place it is likely damage caused by either bug-disease or wrong growing conditions.

First cacti is also too deep inside of pot (second may be too); did you repot it after purchasing? They both should be in well draining potting mix - enough so plant doesn't get 'lost' in container, and drainage holes (maybe your containers have them).

Ya I feel like you may have a little bug issue. Make sure these are kept away from your other plants. I've been lucky enough to never have mites of any kind that I know of :). So I don't want to give you false info on how to treat it. Lastly, I agree with Rina you might want to replant your cactus higher up in the container, and bleach the containers while your at it:)

It looks like flat mite damage. Most likely not corking or sunburn based on your light situation and the spread out location of the damage.

http://www.cactiguide.com/forum/viewtopic.php?f=24&t=32559&p=287283&hilit=flat+mite#p287283