Alcan’s bonds may be a risky bet after Rio Tinto’s big ticket $38 billion deal to buy the company.

The transaction, which Rio will pay for by ramping up debt, will lead to a weaker credit profile for both companies, Gimme Credit analyst Carol Levenson wrote in a note.

The move seems out of character for Rio, known as a conservative company wary of adding to its debt, Levenson wrote. It also has not been particularly acquisitive, and given the size of the deal, the Alcan purchase brings along higher execution and financial risk.

Levenson said that Rio Tinto’s free cash flow was $1.6 billion in 2006 and that Alcan’s was lower, which will make it difficult for the combined company to reduce what she estimates will be a debt load of $47 billion.

Before any stock issuances or divestitures - the companies plan to sell Alcan’s $6 billion packaging business fairly quickly - she estimates credit quality dropping to the mid triple B rating range. “With Alcan 2013 paper seen trading at 90 basis points, and the eventual outcome still far from certain, we would gratefully sell,” she wrote.