* Older crude unit operating normally * New CDU is centerpiece of the expansion HOUSTON, July 19 (Reuters) - A new crude distillation unit (CDU) hit by accidental chemical corrosion last month at Motiva Enterprises' newly expanded Texas refinery is expected to restart early next year, the company said on Thursday. Motiva, a joint venture of Royal Dutch Shell and Saudi Aramco, had previously said the pitted and scarred CDU at its 600,000 barrel-per-day (bpd) refinery in Port Arthur would be shut for several months. On Thursday the company specified an expected restart of the 325,0000 bpd unit in early 2013, but declined to specify which month or quarter. "Normal operations will resume as soon as it is safely possible to do so," spokeswoman Kayla Macke said. "The target timeframe for the crude unit restart is early 2013." The refinery's older 275,000 bpd crude unit was operating normally, Motiva said, as were all seven other expansion units, though some were running at reduced rates. Those include catalytic reformer, delayed coking unit and a hydrocracker. "Motiva is working to optimize operations without the new crude unit," Macke said. Sources familiar with refinery operations told Reuters in mid-June that the unit could be shut up to a year -- more than twice as long as initial estimates -- because of extensive corrosion damage to the unit central to the plant's five-year, $10 billion expansion just five weeks after startup. Those sources also told Reuters in mid-June that corrosion found in vessels and piping within the unit prompted the shutdown. Further investigation, those sources said, showed that a few gallons of a chemical known as caustic inadvertently seeped into the new unit as workers repaired an unrelated valve leak. On Thursday Motiva said its preliminary inspection showed accidental contamination with caustic, also known as sodium hydroxide, "which resulted in cracks in stainless steel piping and other parts of the crude unit." The new CDU is the centerpiece of the expansion that pushed the plant's overall capacity to 600,000 bpd, surpassing Exxon Mobil Corp's 560,500 bpd Baytown, Texas refinery as the nation's largest. Motiva started up the new crude unit in April, and it had nearly reached full capacity by May 31, when Motiva held a commissioning ceremony near the plant featuring Shell CEO Peter Voser and Saudi Aramco CEO Khalid Al-Falih. The unit shut the following week for the valve repair, and during that time the caustic seeped into it and vaporized into a corrosive agent. Getting the crude unit back into production is expected to cost $300 million to $400 million -- about the cost of its initial construction, industry analysts say. Crude units perform the initial refining of crude oil coming into a refinery and provide feedstock for all other production units.