The City of Phoenix has installed over 40 miles of unlined concrete sewers ranging in size from 24- to 90-inches in diameter. These large diameter sewers are subject to flat slopes and warm temperatures which favor the conversion of hydrogen sulfide into corrosive sulfuric acid. Most of the pipelines are key interceptors where there is a high consequence of failure and all of the pipelines are over 35 years old. In order to maintain system operation, the City has proactively investigated these sewers since 1992 to assess the condition and identify rehabilitation needs. This paper identifies the unique conditions being experienced by the City’s collection system and the impact that rapid corrosion is having on the life of the system. Based on ten years of in depth investigation, the primary issues include: • Failures types including uniform crown corrosion, springline corrosion, and invert erosion • Case study investigations identifying springline corrosion in excess of one inch of concrete loss per year • Extreme sewage composition and operating conditions • Primary causes for the increased rate of corrosion • Effect that different pipe manufacturing methods have on corrosion and pipeline deterioration • Methods implemented to inhibit corrosion and extend pipe life including chemical additional, crown spray, and sewer rehabilitation • Failure modes and catastrophic failures Due to the rapid deterioration that is being experienced, the City has accelerate their rehabilitation program to include lining of all unlined concrete pipelines over the next three years at an estimated cost of $140 million.