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I. STAINLESS STEEL MATERIAL

Stainless steel is an alloy of steel in which the main components are Iron (Fe), Chromium (Cr), Nickel (Ni), Manganese (Mn)...

Depending on the percentage of Cr, Ni, Mn... and other additives, stainless steel products have the following lines: stainless steel 201, 208, 304, 316.... Ingredients such as Cr, Ni, Mn creates a protective film for steel against oxidation.

In these compounds, Cr is the main ingredient that creates the anti-oxidation properties of stainless steel, making this product shiny and resistant to rust and yellowing.

II. DOES TIG WELDED 304 STAINLESS STEEL TANK RUST?

1. In an environment without sea salt

In environmental conditions without sea salt, TIG welded 304 stainless steel tanks can still rust due to the following reasons:

- Due to the flat panels production process at the factory, the final steps such as soaking and annealing to remove Fe residue are not thorough, leading to free Fe content remaining on the surface.

- Due to the process of processing flat panels by folding and bending, it causes mechanical damage at the grooves. This leads to some Fe atoms losing their Cr layer to form free Fe appearing on the surface.

⇒ Fe atoms will come into contact with acid (generated from rain) or combine with Oxygen atoms in the air to create FeO, Fe₂O₃. These are the compounds that have a yellow color.

Fe + H⁺ (H₂SO₄, HCl, HNO₃) → FeO + H₂O

2Fe + O₂ → 2FeO

Figure 1. Rusted surface of 304 stainless steel panel

- Due to connecting the panels using TIG welding, the welds are completely inside the tank and in direct contact with water. In clean water, there is still residual chlorine that is highly oxidizing, reducing the activity of Cr leads to yellowing of stainless steel.

2Cr + 3Cl₂ → 2CrCl₃

- At the weld positions, when heating the weld will melt the surface, Fe, Cr, Ni atoms will redistribute on the surface. At welds where Fe atoms have a higher density, these Fe atoms will escape from the Cr protective layer and combine with Oxygen in the air to create a yellowing compound (FeO).

2Fe + O₂ → 2FeO

- In addition, with high water pressure, the inside weld seam is at risk of gradually separating. The welds will have cracks, allowing Oxygen to penetrate into contact with Fe ⇒ The risk of water leakage at the weld seams with water pressure is possible.

Figure 2. TIG welded 304 stainless steel tank is rusted at the welding positions

2. In an environment containing sea salt

- The composition of sea water is relatively complex, in addition to containing compounds such as salt, oxygen, nitrogen, carbon dioxide and some organic substances originating from plants and animals. Especially in sea water, there is at most about 35 grams of mineral salt and it contains 77.8% Sodium Chloride salt (NaCl), also known as table salt. It is known that the average salt ratio, also known as the salinity of sea water, is about 3.5%, but this salinity also depends on the correlation between the evaporation of rainwater and river water from the continents to the sea.

Figure 3. Chemical composition of seawater

- In the marine environment, onshore is often affected by oxidation and rust faster than underwater, because onshore is often affected by wind carrying water vapor and salt that sticks to the surface of the product and then flies away, this process continuously creating a dense layer of salt on the surface of the product that is always wet, thus it has a great impact on the process of oxidation and rapid rust. We often see steel railings or steel building frames near the coast being destroyed quickly due to the above factors, while when underwater, stainless steel is destroyed more slowly because of the dilute salt content dissolved in the water.

- Stainless steel 304 can withstand the corrosion of most oxidizing acids. But stainless steel 304 has a weakness: it is susceptible to corrosion from chloride solutions, or from salt water environments such as the seashore. Chloride ions can create areas of corrosion, called "pitting", which can spread under protective Cr barriers to degrade the internal structure. Solutions with a Sodium Chloride concentration of at least 25 ppm may begin to have a corrosive effect.

- There is no Molybdenum (Mo) in the composition of stainless steel 304 - a metal with significant corrosion resistance, especially in marine or chloride environments.

⇒ Therefore, in a sea salt environment, 304 stainless steel easily corrodes and rusts, affecting drinking and daily life water.

III. WATER TANK SOLUTIONS TO AVOID CORROSION IN SEA WATER ENVIRONMENT

1. 316 stainless steel tank assembled with bolts

- Stainless steel tanks assembled by TIG welding will be corroded by destroying the protective Cr layer during the welding process as mentioned in section II.1. Therefore, the solution will be to assemble with bolts to avoid destroying the Cr layer on the surface of the stainless steel panels.

- Seawater environment will corrode stainless steel 304 as mentioned in section II.2. Therefore, a good solution would be to use 316 stainless steel tank assembled with bolts because of the outstanding features of stainless steel 316:

   + According to conclusions from in-depth research by the Australian Stainless Steel Development Association, stainless steel 316 can withstand the impact of salt in the marine environment. The main components of stainless steel 316 are: Cr=16-18%, Ni=10-14%, Mo=2-3%.

   + The composition of stainless steel 316 contains about 2-3% Molybdenum (Mo). This addition increases corrosion resistance, especially to sea salt, chlorides and other industrial solvents.

   + The corrosion resistance of stainless steel 316 is higher than stainless steel 304. Stainless steel 316 is considered a metal that is resistant to salt water up to 1000 mg/l chlorides at ambient temperature, but reducing to about 500 mg/l at temperatures above 60^oC. In addition, stainless steel 316 is an excellent metal because it is resistant to pitting and crevice corrosion in chloride environments with warm temperatures.

2. FRP/GRP tank with stainless steel 316 accessories

In addition to 316 stainless steel tank assembled with bolts, the FRP/GRP tank option with 316 stainless steel bolts and tie rods is also an effective anti-corrosion option in coastal areas because:

- FRP/GRP tanks with assembled panels of plastic and composite nature are completely not corroded by sea salt.

- FRP/GRP tanks are manufactured according to SS 245:2014, SS 375:2015 standard issued by TUV Singapore to ensure safety for domestic water.

- FRP/GRP tanks are NSF certified by National Sanitary Foundation.

- FRP/GRP tanks meet British WRAS certification for drinking water safety and hygiene.

- Metal accessories in contact with water such as bolts and tie rods made of stainless steel 316 are guaranteed not to rust.