How to avoid omitted namespace on OuterXml

When trying to build SOAP request using XmlDocument
when calling myXmlDocument.OuterXml - it omits the "tns:" which breaks the SOAP request

Simplest code to show the problem follows:

var doc = new XmlDocument();
    doc.LoadXml("<Root />");
    XmlNode node = doc.CreateElement("tns:MyNode");
    string nodeName = node.Name;    // "tns:MyNode"
    string nodeXml = node.OuterXml; // "<MyNode />"
    // Why OuterXml does not show the correct node.Name