Credential Does Not Take Effect

I have a sqlserve, which service account is 'local system', running in machine A.

A credential ,which associated a windows user U1 in machine A, mapped to a sqlserver login Login1.

A file named 1.txt that only can be accessed by U1 in machine A.

A CLR procedure P1 that would read the 1.txt file.



I encounter a error when i access the 1.txt file through P1 as Login1:

A .NET Framework error occurred during execution of user-defined routine or aggregate "HelloWorld":

System.UnauthorizedAccessException: Access to the path 'E:1.txt' is denied.

System.UnauthorizedAccessException:

at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)

at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy)

at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options)

at System.IO.StreamReader..ctor(String path, Encoding encoding, Boolean detectEncodingFromByteOrderMarks, Int32 bufferSize)

at System.IO.StreamReader..ctor(String path)

at StoredProcedures.StoredProcedure1(SqlString fileName)

.



I think it need a credential when a sqlserver login access some resource outside the sqlserver, so i add a credential and mapped to the login

any suggestions would be appreciated.
And please correct me if i have any inaccurate concept.