Bond sales this year by blue-chip companies in the U.S. are about to exceed $1 trillion as investors seeking refuge from negative interest rates underpin a borrowing binge.

Investors are embracing investment-grade debt, which has gained 9.49% this year after losing 0.7% in 2015, according to the Bloomberg Barclays U.S. Corporate Bond Index. Sales in August, which are typically slow due to the summer holidays, topped $115 billion, making it the busiest August in at least 12 years, according to data compiled by Bloomberg. More than $962 billion of the debt has been issued this year, and at that pace sales may reach the $1 trillion mark in September.

Demand for U.S. corporate bonds "remains strong" given "the growing pool of negative-yielding assets in Europe and Japan," said Nathaniel Rosenbaum, a credit strategist at Wells Fargo Securities. "Supply tends to follow demand, so we expect robust issuance heading into the fall."