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Reason of cargo line leakage & procedure for chemical tankers

Galvanic corrosion & pipeline failure
There are many reason that may lead to cargo line failure on board chemical tanker. Galvanic corrosion in the cargo and stripping pipelines may cause several leakage. One of the sources of such corrosion in pipelines is variation in corrosion resistance at adjacent points in the piping. These variations are caused by inadequate surface preparation of the interior of the piping following welding during the vessel’s construction.

The weld seam and the adjacent heat affected zone will suffer from thermal oxidation, seen as dark oxide bands. At these areas the protective chromium oxide layer cannot adequately form, and the resistance to corrosion will be less than that of the surrounding stainless steel not affected by welding.

Below is some example of pipeline corrosion
    line 1
    Fig:line 1 before treatment
  1. Line 1– Extensive rust spots, most likely caused by free iron/oxides trapped on pipeline surfaces


  2. line 2
    Fig:line 2 before treatment
  3. 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 pickled and the upper half circle untreated


  4. line
    Fig:line 3 before treatment
  5. Line 3– Extensive contamination, especially near weld seams, possibly caused by free iron/oxides trapped on surfaces


  6. line 4
    Fig:line 4 before treatment
  7. Line 4 – Weld seams contaminated with significant corrosion products


Diagnosis

Presence of chromium in quantities greater than 12 weight percent. The ability to form chromium oxide in the weld region must be maintained. Some stainless steels are sold containing as little as 9 weight percent chromium and will rust at ambient temperatures.

Stainless steels are subject to localized corrosive attack. 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.

Discoloration of the heat-affected zone is due to absence or lack of use of an inert gas during welding or pickling. Susceptibility of steel to liquation cracking. Occurrence of Cracks in various regions of the weld in the underlying weld metal or adjacent heat-affected zone. Problems associated with discoloration of stainless steel welds are:
  1. Reduction in Corrosion resistance.
  2. Absorption of chlorides causing a corrosive micro environment.
  3. Low amounts of chlorides causing chloride stress-corrosion cracking and pitting.

Recommendation To carryout chemical cleaning (Pickling) of the deck cargo lines and cargo stripping lines. Incomplete or substandard welding within the piping welding cannot be corrected by chemical means.

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.


Results:
line 1 after treatment
Fig:line 1 after treatment
Line 1 (after treatment) – Cleaning and pickling successful
line 4 after treatment
Fig:line 4 after treatment
Line 4 (after treatment) – Pickling successful, some evidence of trapped air pockets caused by vessel trim Conclusion: The cargo and stripping lines of the ships were successfully chemically cleaned. Removal of contaminants such as heavy oxides enables entire stainless steel surface to have a uniform character in terms of chemical composition. Cargo lines shall be thoroughly cleaned with FW water and / or steam each time tank cleaning is carried out to avoid such pipeline corrosion resulting in leakages.





Related Info:

Voyage planning and related considerations

Preparation for cargo operation

Preparing a cargo tank atmosphere

Cargo unloading operation safety precautions

Liaison between ship and shore

Cargo line leakage countermeasures

Checklist for handling dangerous liquid chemicals in bulk

Recommended temperature monitoring equipments onboard

Practical example of solving tank cleaning problems

Pre-cleaning /washing of cargo tanks

Risk & hazards of chemical contamination onboard

Cargo compatibility and reactivity of various chemical cargo

Poisoning and required first aid treatment onboard

Chemical tanker safe mooring practice

Determining presence of contaminants in chemical cargo

Handling various grade liquid chemicals during loading

How to prepare a cargo loading or discharge program ?

How to avoid solidification in cargo tanks ?

Cargo segregation requirement for chemical tankers

How to take cargo samples ?

How to arrange disposal of tank cleaning waste ?

Restrictions on discharge cargo residue into sea

Retention of slops on chemical tankers

Vapour emission control requirement for chemical tankers

Handling self reactive chemicals

Handling of toxic chemical cargoes





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