u/nullfull

I keep seeing 316 specified for sodium hypochlorite because a chart said it was resistant, and then the first pinholes show up at a weld or under a deposit, not on the nice coupon.

The chemistry is unfriendly in two directions: chloride pitting and crevice attack, and the solution getting more aggressive as it ages or gets warm (pH drops). 304 goes first. 316 lasts longer and still pits. Dead legs and heat-affected zones are where I look first.

For people who have actually replaced this stuff: was your first leak a weld, a crevice under a deposit, a valve, or a heat-traced line? And did you fix it by changing alloy, dropping temperature, or changing how you store and dose the bleach?

Plants that were designed as baseload are getting two-shifted or load-followed more than the original spec assumed. The pressure parts usually survive that. The demountable joints are what I see give up first.

A few things that show up after a few dozen thermal cycles:

- Bolt, flange, and body don't grow the same. The preload you put on cold is not the preload you have at temperature, and it is not the preload you have after cooldown either.

- Bolts relax. Especially if they were close to yield at assembly, or if the joint sits hot for a long hold. Next cooldown, the face has less squeeze than you think.

- A joint that was dry on a cold hydro can still weep once it's hot. Ambient leak-tight is not the same test as service temperature. If you only check it cold, you have not qualified the hot joint.

- Cycled joints look different at outage than joints that sat. Witness marks, weep stains below the split line, bolts that take a different torque the second time.

For people who have actually walked a cycling unit: what is your first-to-leak component on a two-shift schedule, and do you re-torque hot or just live with a known weeper until the outage?