Iowa Code § 535.5
If it is ascertained in an action brought on a contract that a rate of interest has been contracted for, directly or indirectly, in money or in property, greater than is authorized by this chapter, the rate shall work a forfeiture of eight cents on the hundred by the year upon the amount of the principal remaining unpaid upon the contract at the time judgment is rendered, and the court shall enter final judgment in favor of the plaintiff and against the defendant for the principal sum remaining unpaid without costs, and also against the defendant and in favor of the state, to be paid to the treasurer of state for deposit in the general fund of the state, for the amount of the forfeiture. If unlawful interest is contracted for the plaintiff shall not have judgment for more than the principal sum, whether the unlawful interest is incorporated with the principal or not.
Iowa Code § 535.5
83 Acts, ch 185, §52, 62; 83 Acts, ch 186, §10109, 10201, 10204