Motor magazine
Oct. 1982  

Solder Bloom --  by Paul Weissler

The quick move by American antifreeze makers to formulations that 
provide good protection against aluminum corrosion has, to date, 
apparently forestalled a problem.  Even the VW antifreeze has 
apparently been successful in this regard.  There do not seem to be a 
lot of radiators being plugged by aluminum particles transferred from 
the cylinder head.

However, another cause of plugging persists; high-lead solder 
corrosion, called solder bloom, in the conventional copper-brass 
radiator.  This corrosion shows up as a white deposit that can 
"bloom" over the tops of the radiator tubes, blocking them.

It's a problem that has nothing to do with the engine but results 
from the type of solder used at the tube-to-header joints, and in 
some cases, the way the solder is applied.  There's no antifreeze 
inhibitor formula that seems to be able to stop it cold for long 
periods.

The problem of solder bloom is also unpredictable.  Some radiators in 
some models years are apparently more prone than others.  In any 
case, the problem usually takes several years to develop, and you can 
see it through the radiator fill neck.

Once you find it, about the only cure is to have the radiator 
cleaned.  The best results are obtained with an ultrasonic cleaner, 
although a radiator shop's boilout tank may work if the cleaning 
solution is up to proper temperature and the caustic concentration is 
adequate.  Never use the two-step oxalic acid cleaning method to 
remove solder bloom.  Although some pour-in radiator cleaners are 
recommended for solder-bloom, the results are inconsistent and are 
poor if the bloom is severe.

The crimped-together and epoxy-sealed aluminum radiators do not 
encounter the solder bloom problem.  However, there durability is yet 
to be proved.  The early Rabbit and Scirocco radiators, for example, 
may be failing at a rate higher than conventional radiators, 
according to the National Automotive Radiator Service Association.  
There failures are primarily perforation-- that is , corrosion of the 
aluminum tubes starting from the inside and coming through.  There is 
some indication that VW aluminum radiators aren't tolerant of cooling 
system neglect.  VW's failure to prescribe cooling system flush and 
fill intervals may be a factor.

-dan