A proper heat treated longitudinal weld of a welded pipe always has a higher corrosion resistance than an orbital weld made on site.

In most cases the corrosion resistance of the longitudinal weld is in level with the base metal.
The weld process is automatic and performed under ideal conditions, with 100% shielding by gas or powder, and Eddy current testing when welding from strip. The heat treatment and pickling processes that follow, improve and complete the good corrosion resistance.

Some of the most common types of corrosion on welds made on site, as well as solutions how to avoid damages due to corrosion are described below.

There are 3 types of corrosion, which are likely to occur in the weld or weld area.

• Pitting corrosion
• Intergranular corrosion in the HAZ,
• Selective corrosion
  

Pitting corrosion due to Cr- and Mo-segregations.

Autogenous welds of Mo-alloyed austenitic stainless steels suffer from segregation, reducing the resistance to pitting corrosion in chloride environments. Higher Mo-contents give more severe segregation. A solution anneal evens out the segregation and a proper heat treatment will give the weld a pitting resistance equivalent to the base metal.

Welding with the addition of a filler metal with higher Cr and/or Mo-content will also have the same effect.

Pitting corrosion due to weld oxide

The weakest points of a stainless steel pipe system are the welds welded on site. At welding chromium rich oxides are formed in the weld area. Depending on weld method and method to apply the shielding gas, the oxide has various degrees of sensitivity to pitting corrosion.

The oxide consists of Cr and oxygen, which means that the underlying surface is depleted on Cr, and thereby more sensitive to pitting corrosion than the pickled base metal.

Sufficient gas shielding, or removal of the Cr-depleted parts with brushing and/or pickling will decrease the risk for pitting.

Intergranular corrosion

Testing the resistance to IGC was worthwhile 20 years ago when carbon contents were above 0.05%. Today most stainless steels have carbon contents below 0.05% and the L-grades below 0.02%, making IGC-testing more or less worthless. 99% will pass this test independent of the manufacturing history. Therefore, heat treatment of welded austenitic stainless steel pipe to reduce the risk for IGC is seldom necessary.

Selective corrosion due to residual ferrite in the weld

When austenitic stainless steels solidify after welding, small portions of ferrite form in the weld, normally in the range of 1-6%. By this means, the risk of hot cracking is avoided.

In some acids, this may cause corrosion unless the tubes are solution annealed. For example, improperly heat-treated seam welds in heat exchanger tubing may be attacked when cleaned with HCl. One way to reduce this risk is to use bead worked solution annealed tubes.

Segregations in a weld structure

The cast structure of a weld is always subject to segregations that occurs during solidification. Segregation is defined as "micro-areas" with different levels of chemical composition.

Areas with lower contents of Cr and Mo are more likely to be attacked by pitting corrosion.

Austenitic stainless steels with Mo-contents above 3% are more sensitive, and in order to improve the pitting corrosion resistance, the weld structure must be either solution annealed or welded with over alloyed filler metal.

Ferrite content in a weld structure

Austenitic stainless steels have a chemical composition that are balanced to give a small portion of ferrite (1-6%) during solidification of a weld. This reduces the risk for hot cracking.

After a standard heat treatment of the tube, the ferrite content of the weld is halved. The base metal is homogenised during the manufacturing process, and the ferrite content is normally 0%. Some aggresive acids (HCl) and solutions may attack the ferrite in the weld.