Just tried to re-true a front wheel for the first time. However didn’t think to lube the nipples first and I have cracked 2 nipples at the top (aluminium ones) were the spoke key engages, the cracks don’t extend down to the rim though. Is it ok to ride on or do the nipples need replacing? BTW wheel is only 1mm out of true now.

Replace them.

Since you’ve gone to the hassle of trying to fix the wheel, you should probably finish the job. But it’s probably not unsafe.

OTOH it’s only going to get worse if you leave it, eventually you’ll have broken nipples corroded permanently onto perfectly good spokes.

good advice, have ordered a few new nipples of ebay.

However after checking again looks like the spoke has siezed on both these nipples, just soaked in WD40 and tried to free them with grips on the spoke and a screwdriver on the nipple head no joy, I will leave overnight and try again.