Brussels - Brussels and Washington will renew their vows during a whistle-stop visit by President Joe Biden to EU headquarters, following a difficult four years under Donald Trump's rule.

But even with all the goodwill and desire to cooperate demonstrated by the White House, transatlantic ties still have their sour issues.

Here are five topics that could still see tempers flare ahead of Tuesday's summit:

- Airbus-Boeing -

The Airbus-Boeing showdown at the World Trade Organization goes back 17 years, with both sides accusing the other of unfairly subsiding their aviation champions.

Legions of lawyers and officials have battled it out in Geneva since 2004, with both Washington and Brussels winning the right to punish the other with billions of dollars in tariffs.

Just weeks after Biden's inauguration, the two blocs announced a truce and suspended the tit-for-tat duties in hopes of reaching an agreement by July 11.

- Trump's steel war -

Despite the arrival of Biden, the EU is still trying to get a commitment from the US to settle their dispute on steel and aluminium that was provoked by Trump in 2018 in his first of several trade attacks against the Europeans.

Targeting the EU and other trading partners, Trump imposed tariffs of 25 percent on steel imports and 10 percent on aluminium using a WTO rule that allows countries to impose tariffs when national security is under threat.

The EU retaliated and slapped levies on Harley-Davidson motorbikes, Levi's jeans, tobacco, corn and rice.

Talks have been underway since May to resolve the dispute, but the US has so far declined to commit firmly to end the tariffs, which are popular in blue-collar states that are key to Biden politically.

- Tech tax -

Facebook, Google, Amazon and other US tech giants are feeling political heat for avoiding taxes and since 2018, several EU nations -- most notably France -- have led the fight to reform global taxation to reduce big tech's mega profits.

France, Spain and other EU nations imposed their own digital taxes targeting the platforms and provoked a storm of tariffs by Trump.

The real estate tycoon also mostly ignored the efforts to find a globally accepted way to tax big tech, but the arrival of Biden brought those efforts -- hosted by the OECD -- suddenly back to life with a proposal to set a global minimum tax on corporations.

Hopes for a breakthrough are pinned on a meeting of G20 minister next month in Venice, Italy.

Big tech's mega profits

- Nord Stream 2 -

The construction of the Nord Stream 2 gas pipeline, led by Germany, is intended to double the delivery capacity of Russian natural gas to Europe via the Baltic Sea, avoiding countries that have caused problems in the past.

Washington and some European countries -- Poland, the Baltic states and Ukraine -- accuse the project of increasing Europe's dependence on Russian gas.

Energy supply provides Moscow with a tool for political pressure and valuable revenues on which the Russian economy almost exclusively depends.

Gas was to start flowing in early 2020 but was delayed by the threat of US sanctions, with Trump also pushing his European allies to buy American shale gas instead of the supply from Siberia.

In a gesture towards Berlin, the US announced at the end of May that it was dropping sanctions against the main company involved, Nord Stream 2 AG, a subsidiary of the Russian giant Gazprom based in Switzerland, and its German CEO Matthias Warnig.

Who will benefit from the Nord Stream 2 pipeline?

- China -

The EU and China announced at the end of December the conclusion "in principle" of a controversial investment agreement that should give European firms greater access to the Chinese market.

However, this agreement, which has been under negotiation since 2013 and was announced without waiting for Biden to take office in January, has offended Washington, which is counting on European support in the face of the systemic rival that Beijing has become.

The implementation of the pact, strongly backed by Germany, is currently frozen.

The EU recently imposed sanctions against China, accused of human rights violations in the Xinjiang region. Beijing retaliated by sanctioning several personalities and organisations in Europe.