Due to its high resistance to corrosion, stainless steel is used in a variety of industries, from cutlery and kitchenware to oil and gas. Various grades of stainless steel exist that are based on the concentrations of constituents such as chromium, nickel, and carbon. In order for a metal to be classified as “stainless steel” there must be a minimum of 10.5% chromium by weight.
When stainless steel is subjected to an oxygen-rich environment, such as air or aerated water, a chromium-rich oxide layer develops on the surface. This outer chromium oxide layer protects the product from corrosion that may occur as a result of scaling and rusting. Prior to the formation of the oxide layer, the stainless steel is subjected to several surface treatment processes to remove surface imperfections and impurities.
These treatment processes include descaling, pickling and passivation. During the pickling stage, the impurities on the surface of stainless steel are removed. The steel is placed in an acid pickling bath consisting of various additives that partially dissolves the outermost layer of the steel product, exposing a fresh, pure surface. Steel pickling baths generally consist of either a single acid, such as hydrochloric acid, or a mixture of acids, such as nitric acid and hydrofluoric acid.
Over time, pickling baths become contaminated with dissolved metals that were stripped from the steel product. As the concentration of dissolved metals increases, the free acid concentration in the bath decreases, causing a decrease in the efficiency of the pickling process. The free acid concentration in a bath indicates the amount of acid available for the pickling process. When this concentration drops below a certain threshold, additions of acid are made to ensure there is sufficient free acid available. The total acid concentration of a bath is the concentration of both free acid and spent acid that is no longer available for pickling. Once a certain limit of spent acid is reached, the bath are drained and replaced with fresh chemicals.
In addition to having quality limits for free and total acid levels, pickling baths must also maintain a certain oxidizing potential. The concentrations of ferric (Fe³+) and ferrous (Fe2+) iron dictate the oxidizing power of the pickling bath. The oxidizing power affects the time required for pickling and the quality of the finished product. If the levels of total iron are too low or too high, or the ratio between ferrous and ferric iron is skewed, the pickling process becomes ineffective. To adjust the ferric to ferrous ratio, oxidizers such as hydrogen peroxide may be added to the bath to oxidize ferrous iron to the ferric form. Generally the ideal ratio of ferrous to ferric iron is 3 to 4.
A stainless steel processing company contacted Hanna Instruments in search of an automatic titrator with the hope of improving analysis of their hydrochloric acid based pickling bath. Previously, the processing company performed manual titrations for the determination of free and total acid as well as ferric and ferrous iron. They were looking to automate and simplify their process with the help of Hanna’s titrator.
Hanna Instruments recommended the Automatic Potentiometric (pH/mV/ISE) Titration System - HI932 with two analog boards, two pumps, and two burettes. For free and total acid determination, Hanna Instruments supplied the Refillable Double Junction pH Electrode - HI1043B for use in strong acids. The customer utilized sodium hydroxide (NaOH) as the titrant and titrated to a fixed endpoint of pH 4.2 for free acid and pH 8.6 for total acid.
For free and total acid determination, two separate methods were created. Using the “Linked Method” option equipped on the HI902C, the “Total Acid” method was linked to the “Free Acid” method, where upon completion of the “Free Acid” method, “Total Acid” would begin either immediately or upon a manual start. Because the customer was utilizing a fixed pH endpoint for two of their analyses, Hanna instructed them to frequently calibrate their pH electrode to three points in order to ensure accurate results.
For the determination of ferrous iron, the customer was previously performing an oxidation-reduction titration to a color endpoint. In this reaction, ferrous iron is oxidized by a potassium permanganate (KMnO₄) titrant. Hanna Instruments supplied the Refillable Combination ORP Electrode - HI3131B for use with the HI902C to automate their titration. Due to better chemical stability, Hanna recommended the customer change their titrant from KMnO₄ to cerium (IV) sulfate (Ce[SO₄]₂), allowing them to standardize their titrant less frequently and maintain accurate results.
After the customer performs their acid and ferrous iron titrations, the concentration of ferric iron is then inferred from the results by subtracting the ferrous iron concentration from the difference between the free and total acid content. The customer was very pleased with the knowledge and technical support they received from Hanna Instruments, and felt confident in their purchase.