On Tuesday, 16 June 2020 21:30:11 UTC+1, Dave Liquorice wrote:
On Tue, 16 Jun 2020 09:12:32 -0700 (PDT), petek wrote:

I have used some zinc yellow passivated screws on my gates to fix the
T&G facing boards to the gate structure.

Steel of any form is useless outdoors. Brass or stainless only, brass
by preferance, stainless is a bit brittle.

I need 4 x 45mm countersunk, posidrive preferably.

Toolstation list 1 3/4" No 8 slotted brass, £8.84/200, unfortunately
OOS for collection or delivery.

B&Q list 4 x 40 £5.37 or 4 x 50 £5.65 brass pozi packs of 25.

What will be the effect, in the short term and long term, of leaving the
screws in place and just going ahead with the Sadolin?

Irespective of the Sadolin they'll rust... Does iroko really need
treatment anyway?

Perhaps I have been extraordinarily lucky, but I really have not had any issues with stainless screws being brittle. Indeed, I have generally found them pretty good.