Pool Chemistry, stainless steel corrosion and prevention, long term study.

Where the ladder tread is mounted, friction can prevent a layer of chromium oxide from forming.

The tread rubs on the rail and damages any chromium oxide layer.

This part looks like it was heated and it looks like mechanical damage.

If the issue was simply water balance, the stainless steel would be more uniformly oxidized.

The selective nature indicates something specific to those areas.

You might have some sort of general galvanic potential difference.

In my opinion, you can have a situation where you have a net inflow of electrons or a net outflow of electrons due to a DC voltage imbalance.

If you have a net inflow of electrons, your metals are protected like in Impressed Current Cathodic Protection.

In this case you get things like copper ions plating out on lights as copper metal.

If you have a net outflow of electrons, your metals will lose electrons and they will be oxidized and corrode at an accelerated pace.

You can get natural rectification of AC to DC.

For example, a heater uses an AC voltage to the flame sensor, which gets rectified to DC due to a surface area difference causing electrons to move mostly one way.

If you have less noble metals somehow connected to the bonding grid, then you can get electrons flowing in and providing protection.

If you have more noble metals connected to the bonding grid, then your bonding grid acts like a big anode and you get a net outflow of electrons.

Aluminum connected to the bonding grid will be an anode to most other metal on the bonding grid.

Much of what you’re seeing is physical damage to the steel surface coupled with structural crevices (places where the different pieces are bolted together). Crevice and pitting corrosion are very prevalent in all types of structure and are self-propagating… they don’t need any outside galvanic driving force. They are a single-metal/spatially-connected type of redox couple. The chloride ions in solution build up in the crevices and pits due to lack of solution flow and the chloride ion itself destroys the passivating layer. Once the iron is exposed to the electrolyte, the Fe2+/Fe3+ redox reaction is self motivated. The simplistic diagram below shows how it happens. Metallic iron exposed to the electrolyte is oxidized in the pit to ferrous iron and water/oxygen gets reduced to hydroxide. Iron metal can also be oxidized to its 3+ state where it readily reacts with hydroxides to form rust. The formation of Fe3+ can also induce the formation of Fe2+ in a redox couple. This is why rust tends to keep propagating into a metal surface even after there is no oxidizer like chlorine or oxygen around. Rust is also not an adherent oxide and there is a huge volume difference between rust and fresh metal so fresh iron is always being exposed as the oxide spalls off.

Long story short … if you want the ladder to last a long time, pull it out of the water periodically and rinse it well with fresh water focusing particularly on the areas where water can get trapped and stagnant.

I have nothing to add that is of benefit to this conversation but I would like to say these discussions are why I follow TFP. Guys like @JoyfulNoise, @JamesW and @mas985 (among others) say it works, I know they've put some thought into it and can back it up with hard numbers.

I'm just here to watch and learn.

--Jeff