Its normal, its typically oxide formation, but that's what wear looks like. Overtime that's what will destroy your tip so always used the lowest heat to get the job done. Its not avoidable but the wear can be minimized. Its the reason I discuss doing basic calibration on your irons to stay as far below 350C as above it, wear will be far faster.
Left is typical wear, bubbles occur when you use too much flux and don't clean it off, or its a poor quality tip so flux eats into it before you can even do anything about it, or too much heat, or a bit of all.
Clean off the flux as soon as possible with the brass wool, use the sponge minimally and use distilled water instead of tap to minimize effects of trace chemicals or salts on your water further affecting the tips life.
And yes, N2 atmosphere can help, but could cost more to maintain that replacement tips on a simple Hakko Fx888.