Corrosion in Sulfuric Acid (H2SO4)
Tantaline® has proven to be the most reliable material available for hot sulfuric acid corrosion media.
The corrosion rate of Tantaline® in sulfuric acid is negligible in any concentration from 0% to 99% up to a temperature of at least 150°C.
At temperatures above 150°C, the corrosion rate of Tantaline® needs to be considered, but the corrosion rate of Tantaline® in high temperature sulfuric acid is magnitudes lower than the corrosion rates of Hastelloy®*, titanium and zirconium. Tantaline® has been applied successfully for more than 15 years in critical customer applications.
Sulfuric Acid Iso-Corrosion Curves
The Iso-corrosion curve here shows the relative corrosion resistance of several specialty alloys and exotic metals in various concentrations and temperatures of sulfuric acid (H2SO4). The corrosive nature of sulfuric acid, as with most acids, is highly dependent on the temperature and concentration of the solution.
Corrosion in Diluted Sulfuric Acid (0-10%)
In diluted sulfuric acid (less than 10%) the aggressiveness of the acid is limited, so austenitic and duplex steels and to some extent titanium may be a cost-efficient option in the low temperature range. At temperatures above 100°C, more corrosion resistant materials like tantalum or Hastelloy®* grades may be required.
Corrosion in Sulfuric Acid in the Concentration Range 10% to 90%
Sulfuric Acid in concentrations between 10% and 90% is known to be especially corrosive. Common type specialty metals like titanium and Hastelloy®* C276 will show rapid corrosion in sulfuric acid at temperatures lower than 100°C. Very few materials including zirconium and tantalum can be applied above the sulfuric acid boiling point. Tantaline® is by far the most economical option. Tantaline® will further maintain its corrosion resistance above the boiling point independent of the concentration up to 99%, whereas zirconium will lose its corrosion resistance in sulfuric acid at concentration above approximately 75%.
Corrosion in Concentrated Sulfuric Acid
Concentrated (water-free) sulfuric acid is less corrosive than watered sulfuric acid and iron based alloys. Silicon cast iron shows low corrosion rates in 99% sulfuric acid, but will suffer from corrosion cracking when in SO3 containing concentrated sulfuric acid (oleum).
Corrosion performance in sulfuric acid
Limitations of Tantaline® in Sulfuric Acid Service
At very high concentrations of sulfuric acid (above 98%), the corrosion performance of tantalum as well as Tantaline® treated parts will decrease due to the existence of "free" SO3. Thus, tantalum in general may not be suitable for fuming sulfuric acid (oleum).
Corrosion in Hydrochloric Acid (HCl)
Hydrochloric acid is a solution of hydrogen chloride (HCl) in water, a highly corrosive, strong mineral acid with many industrial uses.
At lower temperatures, plastic or polymer linings like PTFE or PVDF may be a cost-efficient way to avoid corrosion. At temperatures above 150°C, diffusion through polymer materials becomes significant and a plastic lining is in general not suitable. At these temperatures, the mechanical properties of a polymer lining may be poor, forcing engineers to consider metal based solutions.
Due to the potentially strong reactions between chloride ions and the base metal, the concentration and temperatures of hydrochloric acid needs to be known to determine if the metal can survive and have an acceptable corrosion rate. The iso-corrosion chart below shows the relative corrosion resistance of several specialty alloys and exotic metals in various concentrations and temperatures of hydrochloric acid (hydrogen chloride). Such alloys will typically be applied when stainless steel shows insufficient corrosion resistance in hydrochloric acid. It can be seen that the corrosion attack of hydrochloric acid, as with most acids, is highly dependent on the temperature. Chloride containing acids in many situations show a corrosive nature similar to that of hydrochloric acid at comparable acid and/or chloride concentrations.
Compared with other specialty metals and alloys like Hastelloy®*, niobium and zirconium, the corrosion resistance of tantalum metal is second to none in hydrochloric acid. Tantalum metal is an element (atomic number 73) and is considered to be the most corrosion resistant metal commercially available. At temperatures less than 150°C and a concentration less than 30%, tantalum is considered to have a nil corrosion rate or less than 1 mil/year in chloride acid service.
For precision parts, such as valves, fittings and instrumentation where tolerances are critical to their operation, a corrosion rate 5 mil/year or larger would typically be considered unacceptable in hydrochloric acid applications.
The table here shows the relative corrosion resistance in hydrochloric acid at concentrations in the range of 5% to 35% and temperatures up to 200°C. As can be seen, Tantaline® has superior performance over niobium, zirconium and nickel based alloys.
Corrosion performance in hydrochloric acid
Corrosion in Sour Gas (H2S)
Materials for Sour Gas (H2S)
Sour gas at high pressures, temperatures, and in mixtures of other acids (e.g. formic acids found typically in oil well stimulation environments) is extremely corrosive. In these environments, only a few materials can survive. Tantaline®'s tantalum surface alloys are immune to sour gas environments showing no measurable corrosion rate. This far exceeds the performance of traditional materials like C276 and various grades of titanium metals.
Sour Gas (H2S) and Acid Gas Definition
Sour gas (natural gas) or any other gas that is corrosive due to significant content of hydrogen sulfide (H2S). Natural gas is considered sour if there are more than 5.7 milligrams of H2S per cubic meter of natural gas (4 ppm H2S by volume). The natural gas that contains less amounts of hydrogen sulfide is called "sweet gas."
The terms "acid gas" and "sour gas" are often used interchangeably. Strictly speaking, sour gas is any gas that contains hydrogen sulfide (H2S) in significant amounts, whereas acid gas is any gas that contains significant amounts of acidic gases such as carbon dioxide (CO2) or hydrogen sulfide (H2S). Thus, carbon dioxide by itself is an acid gas but not sour gas.
Corrosion of Sour Gas Under Transportation
Natural gases that have high sulfur and carbon dioxide contents (sour and acid gas) are recognized as being extremely corrosive and dangerous. A special corrosion resistant material will have to be applied, as carbon steel is not suitable. Also, transportation from the wellhead to the sweetening plant must be implemented carefully.
Sour gas corrosion in Elevated Temperature Systems
At elevated temperatures, the hydrogen sulfide acid gas may become severely corrosive. Corrosion rates in the range "inches per year" or "mils per day" have been reported on C276 (Hastelloy®*) autoclaves used in laboratories. In these systems, Tantaline® offers an essentially zero corrosion rate.
Corrosion in Formic Acid (HCOOH)
Formic acid has proven to be highly corrosive at concentrations over 50%, especially if contaminated by acidic oxidizing chlorides. Because of its corrosive nature at high concentrations, material selection can be a challenge.
Stainless Steel, Nickel Base Alloys and Zirconium's Corrosion in Formic Acid
Formic acid is corrosive to many alloys including stainless steel, nickel alloys, and zirconium. Formic acid is a reducing agent, and it attacks stainless steels unless an oxidizing agent is added. Nickel base alloys perform better than SS316, but are attacked if an oxidizing agent is present. Zirconium has been applied successfully in some formic acid applications, but it is sensitive to attack from acidic oxidizing chlorides and other contaminants, such as FeCl3 and CuCl2. Thus, the use of zirconium is not recommended in this media and these materials should be avoided.
Tantaline®'s Immunity in Formic Acid up to 150°C
Tantaline® is the only commercially available metal that is not sensitive to corrosion attack from formic acids in any concentration up to 150°C, where impurities like FeCl3 or CuCl2 may be present.
Corrosion of Tantaline® in Formic Acid Above 150°C
Tantaline® displays very good corrosion resistance in formic acid at temperatures in the range of 150°C to 300°C. In order to estimate the exact corrosion rate, the specific composition of the media has to be evaluated individually.
Corrosion in Acetic Acid (CH3COOH)
Acetic acid (CH3COOH or ethanoic acid) is a colorless liquid with a pungent, vinegar-like odor. Acetic acid is mainly used industrially as a precursor in the production of cellulose acetate mainly for photographic film and polyvinyl acetate for wood glue, as well as synthetic fibers and fabrics.
Although it is classified as a weak acid, concentrated acetic acid is corrosive and attacks the skin. Acetic acid is called Glacial Acetic Acid in water-free conditions and causes severe corrosion issues at elevated temperatures.
The Corrosive Nature of Glacial Acetic Acid at Temperatures Higher Than 120°C
At temperatures higher than 120°C, acetic acids become extremely corrosive, and specialty materials are needed in order to contain them. Zirconium has typically been the material of choice for these practices, since its corrosion resistance to glacial acetic acid is good. While zirconium offers good corrosion resistance to acetic acid, Tantaline®'s corrosion resistance to acetic acid is superb at a cost that is about 30% less expensive than zirconium.
Tantaline® Testing in Glacial Acetic Acid
The test solution was prepared using a reagent grade glacial acetic acid. Three test vessels consisting of new PTFE lined pressure vessels were used. Approximately 65 ml of test solution was poured into each PTFE vessel. A Tantaline® treated part was placed in the first test vessel; the second one had solid tantalum; while the third vessel had zirconium 705 in it. All of the test specimens were completely immersed in the glacial acetic acid and sealed. The assembled test vessels were placed in a laboratory oven at 205°C for a period of six (6) weeks (1008 hours). The test results show that the Tantaline® treated part far outperforms both zirconium 705 and solid tantalum, withstanding corrosion fully.
*Hastelloy® is a trademark of Haynes International