* China Sept aluminium imports up 642% y/y at 356,000 T - customs

* Country was net importer in July, Aug for 1st time since 2009

* Inflows set to fall sharply from Nov after arb closed - analyst

Oct 23 (Reuters) - China reverted to being a net aluminium exporter in September, official data showed on Friday, as the price gap between foreign and domestic metal narrowed, making shipments from overseas more expensive and reducing import volumes.

The world's top aluminium producing country imported 355,999 tonnes of unwrought aluminium and aluminium products last month, the General Administration of Customs said. That compared with exports of 426,469 tonnes.

September imports rose 642% from a year earlier but were down 17.1% from 429,464 tonnes in August, which was a more than 11-year high and the second-biggest monthly volume China has ever imported.

China usually has scant need for foreign aluminium. But its rapid economic recovery from the coronavirus pandemic, compared with the rest of the world, saw the spread between Shanghai and London cash aluminium prices balloon in May, opening up an arbitrage opportunity for cheaper imports.

The window closed in August, but metal booked when it was open has continued to flow in.

China, which was a net aluminium importer in July and August, brought in 1.177 million tonnes in the third quarter overall, according to Reuters calculations, slightly less than exports of 1.195 million tonnes.

CRU analyst Wan Ling estimated that at least 150,000 tonnes of the September imports could be primary aluminium.

"Import volumes will be down significantly from November," even though the arb opened slightly in late September and early October, she added.

In the first nine months of 2020, imports totalled 1.99 million tonnes, customs said.

That was up 381.1% year-on-year and within striking distance of the record 2.32 million tonnes imported in the whole of 2009, when the global financial crisis opened a similar arbitrage and China was a net aluminium importer on a full-year basis.

Customs is due to release a breakdown of imports by origin and type of aluminium and other commodities on Sunday. (Reporting by Tom Daly; Editing by Christian Schmollinger)