Auto increment in alpha numeric value

private void Form3_Load(object sender, EventArgs e)
        {
            var strid = "SEA0000998";
            var intPos = strid.Length;
            var stringChars = new string[4];
            var intSummand = 1;

            for (int i = strid.Length - 1; i >= 0; i--)
            {
                var charTmp = strid.Substring(i, 1).ToCharArray()[0];
                if (char.IsNumber(charTmp))
                {
                    // set the position one element back
                    intPos--;
                }
                else
                {
                    // we found a char and so we can break up
                    break;
                }
            }
            var numberString = string.Empty;

            if (intPos < strid.Length)
            {
                numberString = strid.Substring(intPos, strid.Length - intPos);
            }
            for (int i = 0; i < stringChars.Length; i++)
            {
                strid = strid.Substring(0, strid.Length - numberString.Length);

                strid += (int.Parse(numberString) + intSummand).ToString();

                numberString = (int.Parse(numberString) + intSummand).ToString();

                stringChars[i] = strid.ToString();
            }

            var finalString = stringChars;
        }