Pitting Resistance Equivalent, PREN

As Cr, Mo and N all affect the corrosion resistance of the steel positively, the question may well be asked: How good are the different elements? Is 1% Mo the same as 1% Cr - and what about N?

Comparing Cr, Mo and N is a bit like having your pocket full of money in different currencies. That DKK 100 is more than DKK 75, which is more than DKK 50, is simple enough, but how much is DKK 100 compared to DKK 40 + USD 5 + GBP 8? For this sort of thing you need the exchanges rates between the various currencies after which you will be able to convert to a common monetary unit (e.g. DKK).

Similarly, ”stainless steel currency exchange rates” can be used. If Cr is used as the common monetary unit, dozens of laboratory experiments have shown that 1% Mo protects just as much as 3.3% Cr, and 1% N is just as good as 16% Cr. Thereby, we can develop a kind of 'currency gauge' named 'Pitting Resistance Equivalent' (PRE or PREN). The designation ”PRE” is often used if only Cr and Mo are used in the calculation while ”PREN” is used if nitrogen, N, is included.

% 3.3 % 16 % PREN= Cr+ · Mo+ · N

Two types of stainless steel with the same PREN will have approximately the same resistance against the initiation of pitting corrosion. The higher the PREN, the better, and it is worth noting that theoretically it does not matter whether 1% Mo or 3.3% Cr is added. It is the increase in PREN that is decisive. With Eq. 6.1 as the starting point it is possible to analyse the alloy composition of the steel and on the basis of the content of Cr, Mo and N evaluate how great is the resistance towards pitting corrosion. By looking at the alloy composition in the various types of steel, a chart can be drawn up – a kind of scorecard showing the different steel type's resistance towards pitting corrosion. Such a list of various commercial steel qualities along with their content of Cr, Mo and N is attached in Table 6.1. Note that all data are based on the steels' minimum values for the various, useful alloying elements. Steel of the type 4307 may contain from 17.5% to 19.5% Cr, but it would be too optimistic to rely on the 19.5%. The only thing that is guaranteed is at least 17.5% Cr.

Stainless currency exchange rates

Stainless Steel and Corrosion

EN AISI / UNS / Popular Name Cr Mo N PREN

Ü 2.4819 Hastelloy C-276 14.5 15.0 - 64.0

Ü 2.4856 Inconel 625 20.0 8.0 - 46.4

Ü 1.4547 254 SMO 19.5 6.0 0.18 42.2

Ü 1.4529 904LhMo 19.0 6.0 0.15 41.2

Ü 1.4410 Super duplex; UNS S32750 24.0 3.0 0.20 37.1

Ü 1.4507 Super duplex 24.0 3.0 0.20 37.1

Ü 1.4501 Super duplex; UNS S32760 24.0 3.0 0.20 37.1

Ü 1.4462 Duplex 2205 / UNS S32205 22.0 3.0 0.14 34.1 Ü 1.4539 904L 19.0 4.0 - 32.2 Ü 1.4462 Duplex 2205 / UNS S31803 21.0 2.5 0.10 30.9 Ü 1.4460 AISI 329 25.0 1.3 0.05 30.1 Ü 1.4536 - 19.0 2.5 0.10 28.9 Ü 1.4438 AISI 317L 17.5 3.0 - 27.4 Ü 1.4435 AISI 316L, 2.5-3.0 Mo 17.0 2.5 - 25.3 Ü 1.4432 AISI 316L, 2.5-3.0 Mo 16.5 2.5 - 24.8 Ü 1.4436 AISI 316, 2.5-3.0 Mo 16.5 2.5 - 24.8 Ü 1.4408 Cast alloy: 316 18.0 2.0 - 24.6 Ü 1.4162 Lean duplex 21.0 0.1 0.20 24.5 Ü 1.4362 Duplex 2304 22.0 0.1 0.05 23.1 Ü 1.4404 AISI 316L 16.5 2.0 - 23.1 Ü 1.4401 AISI 316 16.5 2.0 - 23.1 Ü 1.4571 (AISI 316Ti) 16.5 2.0 - 23.1 Ü 1.4521 AISI 444 17.0 1.8 - 22.9 Ü 1.4526 - 16.0 0.8 - 18.6 Ü 1.4318 AISI 304LN 16.5 0.10 - 18.1

Ü 1.4306 AISI 304L (high Ni) 18.0 - - 18.0

Ü 1.4418 - 15.0 0.8 0.02 18.0 Ü 1.4307 AISI 304L 17.5 - - 17.5 Ü 1.4301 AISI 304 17.5 - - 17.5 Ü 1.4509 AISI 441 17.5 - - 17.5 Ü
1.4305 AISI 303 17.0 - - 17.0 Ü 1.4541 AISI 321 17.0 - - 17.0 Ü 1.4371 AISI 202 17.0 - - 17.0 Ü 1.4104 AISI 440A 15.5 0.2 - 16.2 Ü 1.4109 (AISI 430F) 16.0 - - 16.0 Ü 1.4510 AISI 430Ti / 439 16.0 - - 16.0

Chapter 6 - Corrosion of Stainless Steel

EN AISI / UNS / Popular Name Cr Mo N PREN

Ü 1.4034 AISI 440B 14.0 0.5 - 15.7 Ü 1.4057 AISI 431 15.0 - - 15.0 Ü 1.4542 AISI 630 15.0 - - 15.0 Ü 2.4816 Inconel 600 14.0 - - 14.0 Ü 1.4021 AISI 420 12.0 - - 12.0 Ü 1.4003 AISI 410S 11.5 - - 11.5 Ü 1.4006 AISI 410 11.5 - - 11.5 Ü 1.4512 AISI 409 10.5 - - 10.5

Table 6.1: List of the content of Cr, Mo and N in a range of standard stainless steel and the calculated PREN according to Eq. 6.1. For all types of steel the values for Cr, Mo and N are minimum values according to the standards (see Chapter 8). The colour of the dots indicates steel type: Austenitic (red), ferritic (blue), duplex (green), pH alloys (yellow), martensitic (purple) and nickel alloys, austenitic (black).

Again it helps to regard stainless steel as a fishing line. If so, PREN can be regarded as the fishing line's breaking strength, and the higher the breaking strength, the smaller the risk that the line will break at a given load. In principle, choosing stainless steel is therefore 'just' a question of finding a steel with a sufficiently high 'breaking strength' towards corrosion. Unfortunately, a higher level of useful alloying elements also entails a significantly higher price and usually also increased processing costs. The solution to the problem is therefore not to build everything using 254 SMO or super duplex, but to find a suitable balance in between corrosion resistance and price. You have to find out what is required and then you 'just' have to choose steel on the right side of the limit – of course with a healthy safety margin.

Take e.g. the classic 4301 (AISI 304). This type of steel contains min. 17.5% Cr and neither Mo or N which is why PREN₄₃₀₁ will be 17.5. The acid-resistant steel, 4401, contains only 16.5% Cr, but at the same time at least 2.0% Mo, which pushes PREN₄₄₀₁ up to 23.1. Just by observing the alloy composition of the two types of steel, it is possible to predict that 4401 has a better resistance to pitting corrosion than 4301. If you look at the duplex '2205' (4462), PREN₄₄₆₂ ends up at 30.9 as proof of even better resistance against pitting corrosion.

That PREN is not just pure invention can be seen from Figure 6.10 which shows the corrosion resistance for different types of stainless steel in different media. Note that the higher the steel's PREN, the higher the curve for the steel concerned regardless of the chloride content of the water. Steel such as the acid-resistant 4401 is therefore better in all media than the non-acid- resistant 4301 steel. This does not mean that 4401 is immune to pitting corrosion, but that it takes more to initiate pitting corrosion in 4401 than for its younger brother 4301. If 4301 can keep up, so can 4401, and if 4401 suffers from pitting corrosion, so will 4301.

PREN; 4301 v. 4401

Stainless Steel and Corrosion

In the middle there is a middle group of environments where 4401 remains undamaged while 4301 does not – the area between the curves in Figure 6.10. In practice this area is extremely important, and there are numerous examples of the difference between the acid-resistant (4401 category) and the standard stainless steel (4301-) being the difference between success and failure. In Northern Europe, stainless steel installed outdoors will be subject to a harsh salt fog during winter-time. Such conditions are very corrosive towards stainless steel, and while acid- resistant steel (the 4401 class) may be ok, standard stainless of the 4301 class hardly ever is (see Section 7.2 on page 134).

As far as acid-resistant steel is concerned, it should also be noted that the AISI 316(L) group is covered by several different EN standards. It should especially be noted that while 4401 and 4404 both have a minimum of 2.0% Mo, the lower limit for 4432, 4435 and 4436 is 2.5%. The increased Mo content has a positive effect on PREN, and thereby the resistance to pitting corrosion (see Table 6.1). So if there is a working drawing specifying 4432, 4435 or 4436, 4401 or 4404 cannot be used nor can the titanium stabilised 4571. But there are very rarely any problems in replacing 4404 with 4432 or 4436 – apart from the price, of course! The difference between the different standards is described in Chapter 8. So what about the impurities of the steel? As mentioned above, it is particularly sulphur which is able to generate the resistance to corrosion, and if fair is fair S should be included in the PREN equation in Eq. 6.1 – with a great, negative factor indicating that S has a very negative effect. The size of this factor has been much discussed, but ÷123(!) has been suggested for both S and P which gives the following, modified PREN:

% 3.3 % 16 % 123 % % PREN= Cr+ · Mo+ · N- ·

^

h

This means that only 0.1% S needs to be compensated by 12.3% extra Cr or 3.7% extra Mo just to maintain the resistance towards pitting corrosion. For most commercial sheets and tubes, the upper S limit is 0.015% while up to 0.030% is accepted in steel bars and flat steel. This corresponds to a negative effect of 1.85% and 3.70% Cr, respectively. Table 6.1 does not include S which is the direct reason for 4305 being aligned to 4301 although the maximum permissible 0.35% S theoretically is capable of lowering the PREN to under ÷26! Free-machining steel is significantly less resistant to all types of corrosion than the corresponding, sulphur-free qualities. Different types of acid- resistant steel Eq. 6.2: PREN; free- machining steel

Chapter 6 - Corrosion of Stainless Steel