Secure password hashing implementation

I'm implementing a password hashing method for a website. The code below is part of the User class.

Any pointers on what I could do better?

private string GenerateSalt(int SaltLength) {
    using (var rng = new RNGCryptoServiceProvider()) {
        var salt_bytes = new byte[SaltLength];
        rng.GetNonZeroBytes(salt_bytes);
        return Convert.ToBase64String(salt_bytes);
    }
}

private string GeneratePasswordHash(string Password) {
    //create an hmac hash of the password using the pepper value as the key
    using (var hmacsha = new HMACSHA512(ConvertStringToBytes(ConfigurationManager.AppSettings["PasswordHashPepper"]))) {
        byte[] initial_hash = hmacsha.ComputeHash(ConvertStringToBytes(Password));

        //generate a key value using pbkdf2 that will serve as the password hash
        using (var pbkdf2 = new Rfc2898DeriveBytes(initial_hash, ConvertStringToBytes(this.Salt), int.Parse(ConfigurationManager.AppSettings["PasswordHashWorkFactor"]))) {
            return ConvertBytesToString(pbkdf2.GetBytes(128));
        }
    }
}

private byte[] ConvertStringToBytes(string conversionString) {
    byte[] bytes = new byte[conversionString.Length * sizeof(char)];
    System.Buffer.BlockCopy(conversionString.ToCharArray(), 0, bytes, 0, bytes.Length);
    return bytes;
}

private string ConvertBytesToString(byte[] conversionBytes) {
    char[] chars = new char[conversionBytes.Length / sizeof(char)];
    System.Buffer.BlockCopy(conversionBytes, 0, chars, 0, conversionBytes.Length);
    return new string(chars);
}

public void SetNewPassword(string NewPassword) {
    this.Salt = GenerateSalt(64);
    this.PasswordHash = GeneratePasswordHash(NewPassword);
}

public bool ValidatePassword(string AttemptedPassword) {
    return (GeneratePasswordHash(AttemptedPassword) == this.PasswordHash);
}