In the automotive trade corona stain is commonly known as the light brown or rusty stain found on a spark plug operating at normal temperature (or so says a promotional video from NGK) However we tend to beleive that the term "Corona stain" should be used in conjuction with a hard night on the booze (namely "corona") and a visit to the bogger the morning after,hence creating a "rusty or brown corona stain" around the toilet or on ones undergarment that appear to have a hazey "ring around saturn" appearance
Hey johnny,dont go in that bogger it has been totally blown out and has a corona stain around the rim"
by john coffee December 30, 2008