text: This can be prevented by using sodium phosphate () instead of sodium carbonate as softening reagents. What happens is that its molecules will get inside the hair-line crack and block it, as a result of which sodium hydroxide, even if it is there, will not be able to come in contact with iron, and no reaction will be there. Adding tannin or lignin to boiler water blocks the hair-line cracks and prevents infiltration of NaOH into these areas. Adding NaSO to boiler water also blocks the hair-line cracks.