Technical standards recognise only one term: stainless steel. “Acid-resistant steel” – a literal rendering of the Polish stal kwasoodporna – is a colloquial expression that refers to corrosion resistance in general. In market practice the two are often used together, as “stainless, acid-resistant steel”, which is correct as long as a specific material grade is stated.
In the day-to-day work of engineers, sales staff and contractors, material names can be a source of misunderstanding. Requests for quotation regularly mention “stainless steel”, “acid-resistant steel” (in Polish workshop slang, kwasówka) and often a combination of the two – “stainless, acid-resistant steel”.
Is this just saying the same thing twice? Or are they two different materials?
To answer that question, the subject has to be looked at from two perspectives: technical standards and industrial practice.
What the standards say – stainless steel
Formally, the matter is clear-cut. The European standardisation system, in particular EN 10088, uses only the term stainless steel.
For an iron alloy to belong to this group, it has to meet the basic criteria:
- a chromium content of at least 10.5%,
- a chemical composition that allows a passive layer to form.
The same approach is taken in international standards such as ISO 15510.
Conclusion: “stainless steel” is the only term used in the standards.
Why doesn’t stainless steel rust?
The corrosion resistance of stainless steels comes from chromium, which reacts with oxygen to form a thin passive layer (chromium oxide) on the surface of the material.
The most important property of this layer is that it can regenerate itself on contact with oxygen.
It is this mechanism that protects the steel from corrosion. It is worth stressing, however, that the resistance is not absolute and depends on the environment (chlorides, pH, temperature). It also depends on the chemical composition of the steel itself.
“Acid-resistant” – what does it actually mean?
If the standards do not know the term, where did it come from? “Acid-resistant steel” is an expression used in industrial and commercial practice. Importantly, it:
- does not denote a single, precisely defined group of materials,
- is not tied to any specific standard,
- has no clear technical boundary.
In practice the term is applied broadly to various stainless grades from the 1.4… group, including austenitic steels (e.g. 1.4301, 1.4404), duplex steels (e.g. 1.4462) and, in many applications, martensitic steels too (e.g. 1.4034, 1.4028).
In other words, “acid-resistant” steel generally means steel intended for environments where corrosion resistance matters – but the level of that resistance depends on the specific grade.
Are all “acid-resistant” steels the same?
Definitely not. Stainless steels differ in both composition and properties:
- martensitic (e.g. 1.4034) → higher hardness, lower corrosion resistance
- austenitic (e.g. 1.4301, 1.4404) → very good corrosion resistance
- duplex (e.g. 1.4462) → high strength and very good corrosion resistance
These differences are a matter of degree – all these materials remain stainless steels, but their corrosion resistance varies.
PREN – how engineers compare resistance
To compare corrosion resistance more objectively, engineers use the PREN (Pitting Resistance Equivalent Number). In its simplified form:
A higher PREN means better resistance to pitting corrosion. For example, 1.4301 (304) has a lower PREN, 1.4404 (316L) a higher one, and 1.4462 (duplex) a higher one still. Remember, though, that PREN does not define “acid resistance” – but it does show clearly that the corrosion resistance of steel comes in degrees.
Why do we describe our products as “stainless, acid-resistant steel”?
It may look like repetition, but in practice it is a deliberate and functional combination. “Stainless” is the term used in the standards and consistent with the scientific approach to steel. “Acid-resistant”, in turn, is the term our Polish customers readily understand.
The combination reflects the language of the market and covers a wide range of applications. Moreover, it is not misleading – provided the grade is stated.
Golden rule: trust the grade designation! Whatever the name, what matters most is always the material designation, such as 1.4301, 1.4404 or 1.4034. It is the grade that determines corrosion resistance, mechanical properties and suitability for a given application.
Terminology differences across Europe
It is worth noting that the distinction between “stainless” and “acid-resistant” steel is largely a regional matter. Western European countries usually use a single term corresponding to the English “stainless steel” (e.g. acier inoxydable, acciaio inox, acero inoxidable), without any clear terminological split. In Poland and parts of Central and Eastern Europe, on the other hand, several names are used in parallel for the same materials, the result of historical industrial practice and technical language. As a consequence, “acid-resistant steel” plays a supplementary role in these regions alongside the standard term “stainless steel”, and using both together reflects how the industry actually communicates. The phenomenon can be linked in part to the historical development of technical terminology in Central and Eastern Europe, including translations of standards and industrial documentation in the second half of the 20th century, when terms describing the chemical resistance of materials existed alongside the standard nomenclature.
In workshop practice there is also the term kwasówka, a verbal shorthand for stainless steels used in environments that call for corrosion resistance. The term is informal and does not designate any specific material grade.
In summary
“Stainless steel” is the only term used in the standards. “Acid-resistant steel” is a practical expression widely used in the industry. It does not denote a single group of materials but refers to various stainless steels with differing corrosion resistance. The combination “stainless, acid-resistant steel” is correct and functional in commercial communication. What always matters is the specific material grade.
Not sure which steel grade is right for your application?
Contact us – we will help you choose a material for the real operating conditions.
Bibliography
• EN 10088
• ISO 15510
• Davis J.R. (ed.), Stainless Steels
• Sedriks A.J., Corrosion of Stainless Steels
• Callister W.D., Materials Science and Engineering
