Question

For each of the following materials of construction, prepare an approximate plot of temperature versus concentration in water for sulfuric acid and for nitric acid, showing conditions of generally acceptable corrosion resistance: (a) Stainless steel type 302. (6) Stainless steel type 316.

Answer 1

Grade 302

Grade 302 stainless steel is an alloy that, in addition to containing similar quantities of chromium and nickel to grade 304 (18% and 8% respectively), also contains higher carbon levels. Often sold in strip and wire forms, grade 302 stainless steel is primarily used by the manufacturers of conical compression springs. One of the product’s main attributes is that it is resistant to solvents, acids and chemicals. Grade 302 stainless steel is frequently found in spring form within machines used in kitchens, food processing businesses, medical settings and dairies.

Grade 316

The substance contains molybdenum, which lends the product extra resistance to corrosion and oxidisation. This makes it particularly attractive to those looking to use the product in chloride or marine environments. Equally, the substance is ideal for the manufacturing of quality street furniture as it offers real durability along with an attractive finish and an ease of form. Grade 316 is very tough but equally, it has exceptional formability. These twin attributes make it ideal for the production of springs for the marine, oil and gas industries, which simultaneously need both movement and power. Another notable property of grade 316 stainless steel is that it will demonstrate high-level tensile strength, even in very high temperatures, and will not deform permanently under major mechanical stress.

Sulfuric Acid

Sulfuric Acid Of all inorganic acids, sulfuric acid (H2S04) is used in largest volume and is generally considered to be one of the most important chemicals in the industries. Many metallic materials and alloys are corroded by sulfuric acid because of its low pH. In the middle range of concentrated sulfuric acid has the highest concentration of H+ ions, resulting in a strong corrosivity (0.5% H2SO4 + pH = 2.1, 5% H2SO4 + pH = 1.2,50% H2SO4 + pH = 0.3). Depending on the concentration and temperature sulfuric acid can be either a reducing acid or an oxidizing acid. Traces of impurities, e.g. atmospheric oxygen, Fe3+ salts, SO3 etc., can completely change the character of sulfuric acid, turning a reducing solution into oxidizing.

The austenitic Cr-Ni steels achieve their corrosion resistance by the formation of a passive layer on their surface,. This layer can also develop under oxidizing conditions in sulfuric acid, and consists of iron oxide and chromium oxide with incorporated sulfates, which improve its stability. At higher acid flow rates and under reducing conditions, the protective layer is destroyed or its formation inhibited. Sometimes, quite a considerable increase in corrosion is associated with this situation.

Type 316 stainless steel gives useful service at room temperature in sulfuric acid of concentrations lower than 20 per cent and higher than 85 per cent. Between 20 and 80 per cent acid concentration it is subject to rapid attack. At elevated temperatures the corrosion rate increases and even Type 3 16 is not very useful in the acid alone except in very low acid concentrations. However, additions of ferric sulfate, copper sulfate”, nitric acid, chromic acid”: or oxygen greatly reduce attack by sulfuric acid and permit many economical applications of this stainless steel. Thus, nitration reactions involving mixed acid are accomplished in stainless steel equipment. Mixed acid is also stored and shipped in stainless steel containers. Figure 1 illustrates the general sulfuric acid as a temperature. corrosion behavior of Type 316 in function of acid concentration and temperature.

In sulfuric acid at concentrations and temperatures where the usual 18-8 types cease to be useful, stainless steel alloys containing molybdenum with copper and/or silicon, along with higher levels of nickel and chromium, provide good resistance to attack. Thus, an alloy containing 29 per cent nickel, 20 per cent chromium, 2 per cent (minimum) molybdenum, 3 per cent (minimum) copper, 1 per cent silicon, and .07 per cent (maximum) carbon is very useful for handling hot, wet sulfur dioxide containing sulfuric acid in amounts which are too corrosive for the ordinary 18-8 alloys. Other alloys containing 29 to 42 per cent nickel, 20 per cent chromium, 2 to 3 per cent molybdenum, 1.7 to 3.6 per cent copper, 1 per cent silicon and .07 per cent (maximum) carbon are considered to have suitable resistance at 175 F to sulfuric acid concentrations up to 65 per cent and above 85 per cent. In concentrations between 65 per cent and 85 per cent, the most highly corrosive range of sulfuric acid concentrations, they are usefully resistant up to about 125 F and may sometimes be used at somewhat higher temperatures. At boiling temperature they can be used with dilute solutions up to about 10 per cent concentration where the corrosion rate is about 30 mils penetration per year. In fact, in some instances where other materials are present, as in certain oil refinery processes, 45 per cent acid has been successfully handled up to 240 F.

Nifric Acid

Nitric acid is one of the strongest mineral acids and is. also a liquid with vigorous oxidizing properties especially in the anhydrous form. The choice of metals and alloys for nitric acid service is quite limited. For most plant application, the choice is usually between only two general classes of materials - stainless steels and high silicon irons. The choice is further limited because minimum chromium content is required in the stainless steels materials. In nitric acid, austenitic stainless steels containing 18% chromium generally exhibit passive behavior over wide ranges of concentration and temperature. The most important ingredients for resistance to nitric acid is chromium. As the chromium content increases, the corrosion rate decreases. The minimum amount of chromium generally accepted is 18%. This makes the austenitic stainless steels very well suited for practically all concentrations and temperatures . The addition of molybdenum to stainless steels, as in AISI type 316, does not improve corrosion resistance to nitric acid. For lower concentration (<60%), the corrosion resistance of austenitic stainless steels is determined by chromium content. For higher concentrations, with Cr⁶⁺ ions, the corrosion behavior is determined by other elements. At this concentration, the acid shows powerful oxidizing effects. Since the electrochemical potential is near the transpassive region, which increases the corrosion rate.

The austenitic steel AISI type 302L is generally used in nitric acid plants. By reducing the contents of Si, P and S, the corrosion behavior of the steel AISI type 302L (HNO₃ grade) at higher concentrations of nitric acid can be improved. The use of austenitic steels is recommended in nitric acid plant for oxidation of ammonia and further processing to 60% acid.Stainless steels, such as AISI type 302 L, are preferentially attacked by hot nitric acid in the crevices which form, for example, during welding or lining, or between the steel and the plastic sealing discs. While the AISI type 302 L stainless steel corrodes at the rate of 0.05 and 0.07 mm y-' after 100 and 500 h respectively in 15% nitric acid at 367 K, rates of 0.27 and 0.54 mm y-l respectively occur after 500 h in the crevice at the weld and between the steel and the sealing disc.

The austenitic stainless steels have very good resistance to corrosion by nitric acid in all concentrations and practically all temperatures. This corrosion resistance, which is due to the passivating action of nitric acid, permits extensive use of these steels in handling the acid and other chemicals containing it. The process for producing nitric acid by ammonia oxidation owes much of its success to the good performance of stainless steels in the production equipment. Heat exchangers, valves, pumps, and pipe lines for nitric acid are commonly made with these stainless steels. Similar stainless steel equipment is almost indispensable in the production of dyestuffs, high explosives, and in other industries requiring the satisfactory handling of nitric acid. Stainless steel chambers for fuming nitric acid in rockets are a natural result of the good resistance to the fuming acid liquor and vapor, although the rate of attack can be quite high at elevated temperatures.