logout button code...

Use the below code on the page which you want to prevent to be landed using back button

<script type="text/javascript">
  function preventBack() { window.history.forward(); }
  setTimeout("preventBack()", 0);
  window.onunload = function() { null };
</script>

<script language="JavaScript" type="text/javascript">
  javascript: window.history.forward(1);
</script>

Correction in your code:[Additional]

Session.Abandon();
Session.Clear();\\Not required

Becuase Abandon has already thrown your session so no need to clear it.

Regards.. :)

Quote:

I want to disable back button only on login page when user logouts

No, you cannot disable back button of browser in any conditions. You need to rethink your application logic so that it doesn't matter if the user presses the back button. Don't try and mess with the browser. It's either going to only work sporadically, and then, only for those with javascript enabled, and is subject to the whims of those who write the browsers.

The correct method is invalidate the cache, along with server side validation that the session is no longer valid if they try to resend data. To do this, please check Solution 1[^] answer given by Tadit.

[Edit 1]
Added code block.

You can add this javascript to login page.

<script type="text/javascript" language="javascript">
    function fnBackButton() {
        window.history.forward()
    }
    fnBackButton();
    window.onload = fnBackButton;
    window.onpageshow = function (evt) { if (evt.persisted) fnBackButton() }
    window.onunload = function () { void (0) }
</script>

Sometimes, javascript will not be enabled in client browser's. So I would suggest you to clear the cache in login page using c#. Use Page_Init even.

protected void Page_Init(object sender, EventArgs e)
{
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    Response.Cache.SetExpires(DateTime.Now.AddSeconds(-1));
    Response.Cache.SetNoStore();
}

--Amit

Try this:

<script type="text/javascript" language="javascript">
window.history.forward(1);
document.attachEvent("onkeydown", my_onkeydown_handler);
function my_onkeydown_handler()
{
  switch (event.keyCode)
  {
    case 116 : // F5;
       event.returnValue = false;
       event.keyCode = 0;
       window.status = "We have disabled F5";
       break;
  }
}
</script>

code behind:

Session.Clear();
Session.Abandon();

Also refer:
Three ways to disable browser back button[^]
Use Javascript to restrict user from pressing back button in browser[^]
A Thorough examination of browser back button[^]
Simple solution[^]
Disabling browser's back functionality on sign out from Asp.Net[^]
Browser back button issue after logout[^]

..and lot more similar answer on CP QA Forum[^]