Ball won t change direction when hitting a wall

Hello, I’m making a small game and I’ve a problem. I have a unity sphere that is always moving forward. When it his a wall, i’d like it to rotate 90° and keep going forward in that new direction. Being a total beginner in Unity and C#, I’m a bit stuck.
The ball rolls without problem. When it his the wall, i can see the ball rotating but it doesn’t change direction and keeps hitting the wall.
Can someone tell me what is wrong in my code and maybe give me a hint how to fix it? Thank you.

using UnityEngine;
using System.Collections;

public class Boulder : MonoBehaviour {

	public float ballSpeed = 1; 
	private Rigidbody rb;

	// Use this for initialization
	void Start () {
		rb = GetComponent<Rigidbody> ();

	void Update () {
		transform.Rotate (1, 0, 0); // 
		rb.AddForce (0, 0, ballSpeed);

	void OnCollisionEnter(Collision col){
		if (col.gameObject.tag != "Player"){

			rb.transform.Rotate (90,0,0);
			Debug.Log (;

No one knows? :cry: