Good stuff, Vivi - I'm glad to see your P4 is going back into your carry rotation.
I've had a similar experience with the liners on my Police 4.
I think Curlyhairedboy has hit the nail on the head:
curlyhairedboy wrote: ↑Sat Sep 14, 2019 2:34 pm
I think the condensation and evaporation of sweat is a knockout punch on a bunch of stainless steels. You get salt, water, and a bunch of oxygen, cycled over and over and over...
The liners are definitely stainless, not K390 or some type of simple carbon steel.
This is easily confirmed by looking at the polished spine area of the handle, where the backspring, backspacer and the edges of the liners are. On mine there's not a hint of patina or corrosion there, despite constantly being in contact with sweat, vegetable fluids, damp environments etc when being used. Even the most highly polished carbon steels show patina and pepper spotting after some use. If you drop some lemon juice on there, there'll be no reaction.
We also see the question come up from time to time, about whether the liners of a knife are the same material as the blade. Except for the LC200N knives, I don't believe this is ever the case. The properties required for liners are completely different to what are required for a high performance blade.
In general for liners, you don't need high wear resistance, high hardness, or the high carbon content of most bladesteels. Steels like the 420 series in that sense are superior materials for liners than, say CPMS30V or K390.
316 austenitic stainless would be extremely corrosion resistant, but as Sal has referenced hardening and even cryoing their liner material before, I would conjecture that most liners are probably a martensitic 420 stainless, possibly 440 in some cases.
Assuming that everything was done right with the steel grade and heat treatment itself, the issue seems to me that the liners are not highly polished on the inside. I think the machining striations and slight unevenness in the surface on a microscopic level, may interfere with the integrity of the passive chromium oxide layer.
There may also be a slight influence from galvanic corrosion, but on mine I didn't really see that there was a greater amount of rust around the pivot action, where you would expect to have the most contact between the K390 blade and the liner material. There was a fair bit of patina on the liner scales there, but I would expect that.
Also, the skeletonising of the liners probably allows salty/acidic moisture to collect at the edge of the liner holes and gradually wick into the tightly fitted gaps between liners, backspring, scales etc.
It would be interesting to check, but I think the other G10 Seki knives are probably highly polished on the inside surfaces. I don't recall seeing more than a tiny bit of oxidisation on the insides of a G10 Dragonfly I gifted a friend which has had constant daily use for years now.
I should note as well, that my knife was bought second hand, and the first owner carried it in the tropical Northern Territory of Australia. I then used it for over a year constantly, often in the kitchen, and with wet or damp hands before opening it up to have a look.
A couple of previous discussions and pics on the subject:
viewtopic.php?f=2&t=84180&p=1339419&hil ... 4#p1339419
viewtopic.php?f=6&t=58681&p=1338033&hil ... d#p1338033
And of course, they can be cleaned...