Last time I changed my coolant was a one year ago and now it is brown. The engine has less than 20,000 miles on it and about 3.5 years. I put in about 30-40% coolant and two bottles of Water Wetter. It seems there might be some corrosion going on inside. My chrome plated thermostat housing (water neck) has corrosion on the inside on the bottom. It's at least an 1/8 inch deep where it is corroding. When I changed it last I back flushed it so there isn’t much chance that there is much left over stuff from anything previous.