“At-a-glance” corrosion chart for sulfuric acid is based on an extensive survey of construction materials; gives rough indication of suitable alloys.

A variety of methods have been proposed many are used for the presentation of corrosion data in concise form. The authors are basically opposed to the presentation of information in simplified chart form if this material alone is to be used for the selection of the proper alloy of construction.

Corrosion is very complex and its vagaries are numerous. Seemingly unimportant variables such as small amounts impurities can change and presents a bird’s-eye view of the situation and can be used for screening purposes, thus minimizing the number of materials to be tested or considered.

Condensed Data — The question is how far can one go in condensing information and still have it be of substantial value? The chart (p. 170) is an attempt to condense corrosion data so that the general picture can be obtained “at a glance.” A tremendous amount of data has been used as the basis of this seemingly simple graph.

Curves drawn are iso-corrosion lines for 0.020 in./yr. corrosion rates—which are, in most cases, the economic limit of alloy selection.

The chart summarizes limits of usefulness for the more common construction materials in sulfuric acid solutions.

Regions of operating conditions are indicated in which corrosivity is controlled by sulfuric-acid temperature and concentration and the alloys most suited emnomically are shown in this region.

Of coarse alloys capable of handling more aggressive operating extractions can also be chosen if first cost, mechanical or other reasons require this, such as, for example, lead instead of Type 316. silicon-iron heating tubes instead of Hastelloy D in acid concentrators or Alloy 20 impellers to pump warm solutions which ordinarily can be, handled by Type 316 stainless steel when velocity effects play no part.

Shows Inhibitors— Also, the inhibiting effect of metal sulfate salts is shown. This phenomenon substantially extends the useful range of nickel stainless steels.

Dotted lines indicate the average extension of the regions for Type 304, Type 316 and the Alloy 20 series. This extension is very conservatively shown for copper electrolyte solutions when the copper is in the cupric sulfate form; similarly, oxidizing nickel and cobalt solutions may contain enough ferric and cupric sulfates to substantially extend passivity and allow 304, 316 to be employed. This assumes, naturally, that no chlorides interfere with passivity.

Of course, the chart should only be used as a rough guide. And a few comments on the various materials shown on the curves and in the table above are necessary to get a proper perspective. The following discussion is aimed principally at the metal extraction and refining industry, but can be just as useful in any sulfuric-acid-using process.

Old Standbys—Cast iron and carbon steel are widely used handling sulfuric acid in concentration over 70% at ambiest temperatures.

Chemical lead (0.06% Cu) is the famous old standby in sulfuric acid plant construction and ; its merits have gained wide recognition. It resists sulfuric acid very well, except for the strong acids (over 80%) at elevated temperatures.

Lead, cast iron and carbon steel all depend for their corrosion resistance on a protective sulfate film. Solubility of these films depends on temperature and concentration of the sulfuric acid. The films can also be readily destroyed by velocity and impingement conditions, making such materials unsatisfactory for valves, pumps, and often elbows and nozzles.

Stainless Steels —The stainless steels are hot resistance to sulfuric acid environments and the minimum stainless steels we can consider are the 300 series or so-called 18-8 austenitic stainless steels.

Types 304, 321 and 347 can normally be used only in very dilute sulfuric acid applications at room temperature, and show equally good resistance to corrosion. Type 316 has much better resistance to dilute sulfuric and can be used in a broader range of temperatures and concentrations.

Corrosion rates decrease for stainless at high acid concentrations (over 80%).

Where welding is involved, the low-carbon materials (0.03 % maximum carbon) or the stabilized stainless steels are the correct alloy selection to avoid intergranular corrosion. Generally speaking. Type 316L is the preferred and necessary selection for the majority of electrolyte handling equipment.

The Alloy 20 materials are good for velocity conditions and are particularly resistant to solutions containing oxidizing salts, such as ferric and cupric sulfates. Limiting concentrations and temperatures will be extended by the addition of such oxidizing salts to sulfuric acid solutions.

Copper, High Nickel—Copper, silicon bronze and Monel (nickel-copper) find very wide use in chemical processes involving sulfuric acid under air-free and essentially reducing conditions at moderate velocities. These alloys are employed in concentrations generally below 60% sulfuric acid and up to 200 F. Monel especially provides excellent service under these conditions and is commonly used in pickling operations and sulfonation.

In air-saturated sulfuric or in the presence of oxidizing cupric and ferric salts, the corrosion rate will increase considerably for this group.

Ni-o-nel, Labour R55, Illium G, Hastelloy alloys C and F cover somewhat the same, fields of application as the Alloy 20 series— the additional alloy content ensures passivity under more aggressive conditions caused by velocity or temperature. See Chem. Eng., Dec. 14, 1959, p. 194, for data on a new Illium alloy.

Hastelloys B and D were developed for handling sulfuric acid under very severe conditions, and they find use particularly in acid evaporators where dilute sulfuric is concentrated to about 65%. Alloy B is more resistant to boiling sulfuric up to about 60%, while D is employed chiefly over 60% concentration.

Hastelloy alloy B, however, is not recommended for sulfuric solutions containing strong oxidizing agents.

Chlorimets 2 and 3 are widely used in pumps and valves. Chlorimet 2 is preferred under reducing conditions, while Chlorimet 3 performs especially well when oxidizing salts are present in the sulfuric acid solution at moderate temperatures (Behavior of Chlorimet 3 and that of Hastelloy alloy C are comparable).

High Silicon—The high-silicon irons, such as Duriron and Corrosiron exhibit excellent resistance to all concentrations of sulfuric acid up to and including boiling temperatures. They are available in cast form only.

Silicon irons are hard and difficult to machine and are very susceptible to thermal and mechanical shock. These are the chief limitations of this alloy, offsetting in many applications their first-cost price advantage.

Tantalum, Titanium—Tantalum equipment has been used safely with sulfuric acid under a wide variety of conditions without loss or damage due to corrosion. The material gives good performance under reducing as well as under oxidizing conditions. Its price structure, however, limits use to special applications, such as bayonet heaters and liners of steel equipment handling boiling sulfuric acid (concentration by evaporation).

Titanium is chiefly known for its excellent corrosion resistance to a wide variety of oxidizing chemicals, such as nitric acid. It resists sulfuric in the presence of oxidizing salts, such as cupric and ferric sulfates.

Titanium resistance normally breaks down in a reducing system. In a recent development, titanium has been alloyed with a fractional percentage of platinum or palladium which markedly improve its corrosion resistance to reducing acids without impairing resistance under oxidizing conditions.

Karbate, Glass—Karbate and impervious carbon and graphite are suitable for sulfuric acid concentrations from 0 to 90 % and process temperatures up to 84° F. These materials have high thermal conductivity and are practically immune to thermal shock.

Their chief limitation is low mechanical strength. Careful handling is required.

Glass has excellent resistance to sulfuric acid under reducing as well as under oxidizing conditions, and can be used under conditions of temperature and concentration. In commercial applications, it is used either in glass form for transfer lines etc., or in glass-lined steel equipment.

The chief limitation, of course is its mechanical properties. Glass equipment can be easily damaged by mechanical and thermal shock which makes its reliability and justification often questionable.

Plastics, Rubber—Last, but not least, are the plastic and rubber materials.

Piastics in general have good corrosion resistance against dilute solutions, are easy to install and have good electrical insulating properties. However the have relatively low temperate limitations, high thermal rates of expansion and can be subjed to “weathering effects.”

Soft or semi-hard rubber linings handling 50% sulfuric acid room temperatures, and neoprene handling 70% at 120 F are quite popular in sulfuric service.

As for coatings, baked-phenolic and coal-tar materials are usually recommended because of good resistance to sulfuric corrosion, abrasion and erosion. However, pinholes or cracks can result in extensive damage—mod coatings are used on the outside of equipment and structure (noncritical areas) for this reason.