Pitting resistance in stainless steel is controlled mostly by the alloy composition as stated in FAQ 1b. The key environmental variables which influence the severity of an environment's pitting propensity are pH, chloride concentration, and temperature. Their effects are approximately additive and linear.
As a rule of thumb:
increasing temperature 25C is equivalent to lowering pH by 5 or increaing chloride concentration by a factor of ten. The effect of any of these would be offset by an increase in the PREN of 5, which in turn would represent an increase in pitting potential of 250 mV.
In addition, but less quantifiable, microbiological organisms can be harmful, increasing flow rates is beneficial,and adding inhibiting cations is beneficial.
A harmful contaminant in the stainless itself is sulfur. Each 0.001% of sulfur negates the benefit of 1.0% chromium.
It is a common contaminant, especially in TIG welded tubing, where it makes welding easier. It can be kept below 0.001%.
