The pledge or hypothecation to any association of all or part of an account in joint tenancy signed by any tenant or tenants whether minor or adult, upon whose signature or signatures withdrawals may be made from the account, shall, unless the terms of the account provide specifically to the contrary, be a valid pledge and transfer to the association of that part of the account pledged or hypothecated, and shall not operate to sever or terminate the joint and survivorship ownership of all or any part of the account.