Sulphates can cause some serious corrosion issues in boilers and cooling towers across various industries, including power plants, manufacturing, and even oil and gas. This corrosion can lead to equipment failure, downtime, and even safety risks. So, it's crucial to monitor and control sulfate levels.
There are three key methods to determine sulfates content:
- Gravimetric
- Turbidimetric
- Ion Chromatography.
The Gravimetric method involves precipitating sulfates as barium sulfate and weighing the precipitate. The Turbidimetric method measures the turbidity of the sample after adding barium chloride. Lastly, Ion Chromatography separates and detects sulfate ions using specialized equipment.
These methods help you determine the sulfate content in your wastewater and feedwater, enabling you to take corrective action to prevent corrosion and ensure the longevity of your equipment.
The gravimetric determination is not suitable for water samples containing significant levels of calcium and magnesium ions (>20 mg/L as CaCO3), as these ions can also react with EDTA and interfere with the test results. Therefore, this method is generally recommended for water samples with low hardness levels, typically less than 20 mg/L as CaCO3
Outline of the method – a measured excess of Std barium chloride solution is added to the sample and the excess of barium chloride is estimated by titration against EDTA.
Reagents
1. Dil HCL acid – 0.5N
2. Std barium chloride sol (0.04N ) – dissolve 4.886g of barium chloride dihydrate in distilled water and make up to 1 litre. One milliliter of the solution is equivalent to 1.92mg of sulphate (as SO4)
3. Buffer solution – dissolve 67.5f of ammonium chloride in 570ml of ammonium hydroxide (Sp Gr 0.92) and dilute with distilled water to 1 lit.
4. Erichrome black T indicator solution.
5. Std EDTA sol 0.02N
Procedure –
Pipette 10ml of the sample into a porcelain dish and acidify with 1ml of dilute HCl. Add exactly 10ml of standard barium chloride acid. Add exactly 10ml of Std barium chloride solution and stir well. Add 1ml buffer solution and several drops of Erichrome black T. Titrate with Std EDTA solution to a blue colour which does not change on addition of further drops of indicator solution.
Calculation
Sulphates (as SO4), mg/l = 192(10-V/2)
Where
V = volume of Std EDTA solution
Sulphate-Reducing Bacteria (SRB) corrosion is a type of microbiologically influenced corrosion (MIC) that occurs in environments with high sulphate concentrations, typically above 100 mg/L. This type of corrosion is particularly prevalent in industries such as oil and gas, water treatment, marine engineering, and construction.
When sulphate-reducing bacteria break down them and produce hydrogen sulphide gas (H2S). This gas reacts with metals, particularly iron and steel, to form corrosive compounds that can lead to pitting corrosion, crevice corrosion, and MIC. The rate of corrosion can be significant, with reported rates of up to 10 mm/year in severe cases.
To prevent SRB corrosion, it's essential to monitor and control sulphate levels. The American Petroleum Institute (API) recommends maintaining it below 50 mg/L to minimize the risk of SRB corrosion. Additionally, the use of corrosion-resistant materials can help prevent SRB corrosion.
The following ranges are generally considered to be indicative of SRB corrosion:
- Sulphate concentrations: 100-500 mg/L
- Hydrogen sulfide gas concentrations: 1-10 ppm
- Corrosion rates: 1-10 mm/year
- pH levels: 6.5-8.5
Graph showing ranges of sulfate contents in various sources of industrial wastewater:
 |
| Minimum and Maximum in mg/L of sulfate content |
Please note that these ranges are general estimates and can vary widely depending on the specific industry, process, and location