Pitting has occurred on the bottom plate of the newly installed stainless steel tank
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Two new storage tanks for waste phenol were constructed, with a capacity of 1,500 m3 each; the material used for these tanks is S30403. Construction on the tanks began in October 2019, and the main construction work was completed in December 2019. Hydrostatic testing started in October 2020, using river water as the testing medium. The hydrostatic testing was completed in December 2020, after which water was drained from the tanks. On February 15, 2021, while cleaning the bottom of the tanks to remove residual water from the testing process, a thin layer of sludge and sediment was found there. After cleaning it away, it was discovered that there was corrosion in multiple areas on the bottom of the tanks; the corrosion took the form of pitting, with pinhole-like defects, and dark brown stains could be seen on the outer surface. The depth of the corrosion ranged from 1 to 4 mm. Please see the pictures for details. What do you think caused this situation? As a supplementary note, was there any sign of corrosion on the tank bottom plate before the pressure test?
The possible causes of this problem are preliminarily identified as follows: 1. The test medium is river water; it is necessary to check whether the chloride content in this medium exceeds the permissible level, as an excess amount of chloride could be the cause of corrosion. 2. Problems with the stainless steel S30403 base material; 3. Proximity to the sea leads to salt corrosion in the air, and the accumulation of water on the bottom of the tank along with the humid environment cause corrosion of that bottom surface. 4. There is water rust in the water; this water rust causes carburization with stainless steel, resulting in a chromium-deficient condition
Have you heard little about the impact of microorganisms on stainless steel 304L? It really does seem strange that so much corrosion can occur in just 2–3 months
Have you noticed that the places where welding slag splashed were all the ones that had problems?
This stainless steel has been contaminated by carbon steel; maybe there are too many iron ions in the water? ? ?
It’s quite reckless to use river water for testing stainless steel equipment in the first place. Moreover, I saw that the water was left in the equipment for 2 months, and it wasn’t until more than 4 months later that efforts were made to clean the bottom and any remaining water – what you’re doing here isn’t a test of water pressure; it’s a corrosion test. It’s probably mainly a problem caused by chloride ions in the river water, but there are also quite high levels of other elements in it as well; overall, it would be strange if there were no issues. Regarding the specific forms of corrosion, you can search on Baidu for information on sub-surface corrosion of stainless steel; under the special conditions present in certain areas covered by deposits, corrosion occurs more rapidly and in a more complex manner.
Another possibility is that the bottom of the tank was not cleaned properly before the hydrostatic test; there were weld slag or other debris in certain areas, and over the course of 4 months under the influence of river water and sediment, pitting corrosion occurred. In any case, there are many such possibilities.
That isn’t contaminated by carbon steel; it’s spatter from previous welding, as well as contamination caused by plasma cutting in the past. But corrosion has also occurred in some areas that are not polluted, which is really strange and hard to understand; there are many possible reasons for this. Theoretically, corrosion starts at the weakest point, so it begins at the contaminated area.
Yes, there are quite a few reasons. At that time, the property owner said that the chlorine ion level in the river water had been tested and was within the acceptable limits. Since it is a newly built unit, there is no production water or desalinated water available inside; therefore, the owner ordered that raw water be used. Moreover, the construction progress of the unit has been slow, and when cleaning the bottom of the tank, it was discovered that corrosion had occurred at such a rapid pace. We have built many such storage tanks in the past, but this is the first time we have encountered this problem. There are various possible reasons for this, and we are currently analyzing them one by one
Water used for the hydrostatic testing of austenitic stainless steel containers should be drained and dried immediately after the test is completed. When complete drying cannot be achieved, the chloride ion concentration should be kept at no more than 25 mg/L.