Leaks in cargo piping
The vessel was built in 2005, and had, in its first few months of service, suffered several leaks in the cargo piping and in the cargo stripping lines. It was suspected that a possible cause of the leaks was galvanic corrosion. One source of galvanic corrosion in stainless steel piping is variation in corrosion resistance at adjacent points in the piping.These variations can be caused by inadequate surface preparation of the interior of the piping following welding du-ring the vessel’s construction. The weld seam and the adjacent heat affected zone (the “HAZ”) will suffer from thermal oxida-tion, seen as dark oxide bands. At these areas the protective chro-mium oxide layer cannot adequately form, and the resistance to corrosion will be less than that of the surrounding stainless steel not affected by welding.Using a borocope VECOM inspected the stainless steel (AISI 316L) cargo pipe-lines on motorvessel NS Silver and her three sisterships. Subject of the inspection was the dubious state of the welds and discoloration of the heat-affected zone.
Left: Line 1 – Extensive rust spots, most likely caused by free iron/oxides trapped on surfaces.
Right:Line 2 – Evidence that at one time the piping had been half submerged in a pickling acid, with the lower half circle of weld seam pick-led and the upper half circle untreated: 
Left: Line 3 at portable pump connection – weld seam contaminated and showing signs of exten-sive corrosion products
Right: Line 4 – Extensive contamination, especially near weld seams, possibly caused by free iron/ oxides trapped on surfaces:
Diagnosis
The stainless properties of stainless steels are primarily due to the presence of chromium in quantities greater than roughly 12 weight per-cent. This level of chromium is the minimum level of chromium to ensure a continuous stable layer of protective chromium-rich oxide forms on the surface. The ability to form chro-mium oxide in the weld region must be main-tained to ensure stainless properties of the weld after welding. In commercial practice, however, some stainless steels are sold containing as little as 9 weight percent chromium and will rust at am-bient temperatures. Stainless steels are subject to several forms of localized corrosive at-tack. The prevention of corrosive attack is one of the concerns when selecting base metal, filler metal and welding procedures when fabricating components from stainless steel. 
Left: Line 5SB at portable pump connection (before treatment) – weld seam contaminated and showing signs of extensive corrosion products
Right:Line 5SB (before treatment) – Weld seams conta-minated with significant corrosion products.
Discoloration of the heat-affected zone indicates the absence or lack of use of an inert gas during welding or pickling and passivation of the stainless steel after-wards. Another concern when welding the (austenitic) stainless steels, is the susceptibility to liquation cracking. Cracks can occur in various regions of the weld with different orientations such as microcracks in the underlying weld metal or adjacent heat-affected zone.
On the NS Silver VECOM inspectors found welds of different quality and welds with embed-ded slag and undercut welds. In general, weld seams found near the cargo pumps were fully discoloured in the HAZ. The cargo Lines on the back of the deck however were less affected. In addition, several cargo Lines were grossly contaminated by heavy, adherent, particulate debris. Samples of this debri were taken for further analysis by the VECOM laboratory. Discoloration of stainless steel welds may cause serious problems. First of all it reduces the corrosion resistance by preventing a passive protective chrome layer to form. Secondly, as these areas are more porous, they tend to absorb chlorides causing a corrosive micro envi-ronment within the heat-affected zone. Even low amounts of chlorides may cause severe corrosion problems on stainless steel surfaces such as chloride stress-corrosion cracking and pitting corrosion. For use in a marine environment Duplex stainless steels are usually recommended as they have a higher resistance to acids and aqueous chlorides.
Recommendation
The report’s recommendation was to perform a chemical cleaning of the deck cargo lines and cargo stripping lines The effect of this chemical cleaning (pickling) is to remove the oxides at the heat affected zones along the pipinglength where welding was done, thereby allowing the steel surface to come into contact with oxygen from the air, and form the protective chromium oxide layer that gives stainless steel its characteristic corrosion resistance.
Physical faults within the piping, such as incomplete or substandard welding, cannot be corrected by chemical means. The work involved circulating aqueous solutions of degreasing compounds, fresh water, aqueous solutions of pickling acids, and again fresh water, through the piping. In order to achieve this, it was necessary to divide the work into 4 “systems”. Each system, or loop, comprised one or more cargo lines and the associated stripping lines. VECOM was asked by the Superintendent to complete the job in just 14 days.
Conclusion
The cargo and stripping lines of all four ships. were successfully chemically cleaned.
The result of the removal of contaminants such as heavy oxides from the HAZ and also other contamination is that the entire stainless steel surface has a uniform character in terms of chemical composition.
Left: Line 1 (after treatment) – Cleaning and pickling successful.
Right: Line 4 (after treatment) Pickling succesful, some evi-dence of trapped air pockets caused by vessel trim
By maintaining a clean stainless steel surface, and by thorough removal of chlorides derived from seawater used for cleaning (using a thorough follow-up cleaning using fresh water) the oppo-rtunity that galvanic corrosion will occur is greatly reduced. Maintenance of clean and chlo-ride-free surfaces cannot be over-emphasized when discussing the passivity of stainless steels. It is not possible to entirely remove all crevices or crevice-like phenomena. In such areas the possibility for corrosion exists. However by keeping the areas clean and exposed to oxygen, the stainless steel will maintain its resistance to corrosion over a prolongued period of time. The vessel NS Silver experien-ced fewer cargo/ stripping line leakages either during pickling or during subsequent pressure testing with water than was the case for the other vessels to date. The overall condition of the welding when seen using the boroscope appeared better than that seen in reports on other vessels in the series. In the case of mv NS Silver there was a need to trim mv NS Silver more than 7m forward to expose the rudder for essential repair. The consequence of this is that
Photos and boroscope inspection of line
5SB – successful pickling
lines 4, 5P, 5SB and Slop could not be properly drained of liquid, since, unlike lines 1 and 2, there is no connecting line/ valve between the cargo line and stripping line where the cargo line tee is positioned at the manifold. We recommended, therefore, to allow for a proper drain of liquids from the other lines, a ½ “ socket be added as indicated in this photo taken of line 4: 