Italian Prime Minister Giovanni Spadolini arrives in Washington Tuesday for a six-day official visit during which discussion of the Reagan administration's controversial Soviet pipeline sanctions against Western European countries is expected to be high on the agenda.

The prime minister, 57, who will be accompanied by Foreign Minister Emilio Colombo, will meet Wednesday with President Reagan and Secretary of State George P. Shultz.

Questions to be discussed, along with the pipeline, will be East-West relations, Poland, Afghanistan, disarmament and recent developments in the Middle East.

The visit, the first by an Italian prime minister since January 1980, comes two weeks after an Italian company became the first European firm to have energy-related equipment purchased in the United States seized by U.S. Customs as part of Reagan's plan of sanctions against companies selling pipeline parts to the Soviet Union.

Last month Colombo told the Italian parliament that the Oct. 15 seizure of gas turbine parts bought from General Electric by Nuovo Pignone of Florence had "no justifiable motive."

Nevertheless, because of close cooperation on recent NATO strategic decisions and in the new peace-keeping force in Lebanon, U.S.-Italian relations are currently flourishing.

In recent months the Italian government has been taking advantage of its friendly relations with Washington to play an increasingly active role in seeking to ease growing tensions between the United States and other Western European countries.

During their stay in Washington, Spadolini and Colombo will also meet with Treasury Secretary Donald T. Regan, Commerce Secretary Malcolm Baldrige and Deputy Secretary of Defense Frank Carlucci.

The issue expected to attract most attention is that of European-American relations and, in particular, the controversy surrounding the sanctions imposed by Washington on companies selling equipment for the Soviet pipeline to protest the imposition of martial law in Poland.

The Italian government is still hesitating over an unsigned contract with the Soviet Union for massive imports here of Siberian natural gas.