Brian,
Corrosion of metal is accelerated by several factors including type of metal, lower pH, higher concentrations of an oxidizer (dissolved oxygen or disinfecting chlorine) and higher conductivity (and in the case of stainless steel and other metals that form a passivity layer, the concentration of chlorides and sulfates is also significant). The disinfecting chlorine in a pool is by far the most powerful oxidizer -- much more so than the dissolved oxygen present in the water. Nevertheless, the active oxidizer is hypochlorous acid and its concentration is significantly reduced in the presence of Cyanuric Acid (CYA) as the CYA combines with it to form relatively inert compounds.
So while the statement that chlorine levels above 3 ppm can be corrosive to metal is true when there is no CYA present, in the presence of CYA 3 ppm Free Chlorine (FC) is generally not corrosive. Of course, it depends on the metal. It's still corrosive to galvanized steel (that is, zinc-coated steel), but not to stainless steel. And there is always corrosion occurring so what is relevant is how fast it occurs. The bottom line is that for the kinds of materials used in pools, metal corrosion is slow when there is CYA in the water and the TDS (which relates to conductivity) isn't high.
3 ppm FC with 50 ppm CYA has the same amount of hypochlorous acid (disinfecting and oxidizing chlorine) as 0.05 ppm FC with no CYA. That makes the corrosion rate about 60 times slower (or more, for non-linear processes). So yes, I would could consider it irresponsible to make a blanket statement that 3 ppm or higher FC leads to corrosion.
Where we have had reports of very rapid corrosion of stainless steel in less than a year was in a couple of indoor pools that had 3-5 ppm FC with no CYA and also had an SWG so had 3000 ppm salt levels (so conductivity was 3-6 times higher than in most non-salt pools). Combining the effects of no CYA with higher conductivity (and higher chloride levels) led to corrosion of even marine-grade stainless steel. So making blanket statements without understanding the corrosion factors is irresponsible (or careless and misleading as duraleigh said).
As for the recommendation of 1-3 ppm FC with 50-75 ppm CYA, we have several users on multiple forums report that levels below 3 ppm FC with their SWG and high CYA (usually 60-80 ppm) resulted in algae. So that is why I recommend an FC level of around 4.5% of the CYA level which for 60-80 ppm CYA is 2.7 - 3.6 ppm. This lower 4.5% level compared to the minimum 7.5% level in the chart is, as duraleigh says, due to the more consistent dosed levels from the SWG and also from the fact that a portion of the pool water going through the SWG cell is superchlorinated at high chlorine levels and acidic conditions near the chlorine generating plate (so high disinfecting chlorine levels -- manually dosed chlorine is basic/alkaline so the peak disinfecting chlorine levels are more limited). Of course, superchlorination in the SWG cell only works for free-floating algae and does not work against biofilms or algae stuck to pool surfaces.
A more complete technical discussion of hypochlorous acid levels in the presence of CYA (based on science going back to at least 1973) is discussed in
this thread.
Richard
Ok, not really but I appreciate having this explained to me.