Current armour and equipment system in my opinion states heavy unbalance among classes, as DD loaded with 6 T5 armour pieces has really bigger survivalability, when for a high tier BB armoured same way that is not so big advantage.  Moreover, time limit of armours combined with it costs and low creds per battle make it hard to maintain having well armoured ship all the time.

For the cost issue, it would be enough to increase armour cost, but make it unvanishable over time. 

I just think that making armour permanent is just not enough change.

About the game balance, as even the 6 slots sometimes are not enough, there i propose a change:

•  At first, separate armour from support equipment. Current slot should be left for suppot items - with big increase of unlock price, as there will be less things to put into.
•  Then, make separate Armour tab like current Aircraft system. Every CV ship has possibility to load few types of aircraft, which together must sum up to a certain limited amount.  Every plane takes some place. So, make Armour the same way: 

-Every ship would have overall space for Armour. In setting it, there is possibility to really differ defence among ships and shipclasses, more noticeably for players than current ,,defence lvl" etc. Lets say, a DD1 would have armour limit ~25, when high tier BBs may have it counted in hundreds.

-Unlike the aircraft, Belt and Deck armour may be increased 1 by 1, while single bulge or bulkead would take more space, differed by a ship. 

-1 Deck /Belt would refer to 100 dmg def, bulge 100x space taken.

• You can keep variety of steel quality of armour giving better tier armours just a multiplier for the above.