Ржавчина и коррозия: в чём разница и почему это важно для подготовки поверхности

Rust and corrosion are often used interchangeably, but they are not the same thing. This article explains the difference, the main corrosion types that affect steel structures, the ISO 8501-1 rust grades used to describe steel before preparation, and what each means for surface preparation before coating.

What is corrosion?

Corrosion is the chemical or electrochemical reaction between a metal and its environment that degrades the metal's properties. The environment provides the oxidising agent, most commonly oxygen dissolved in water, or water together with atmospheric oxygen. The metal is oxidised (loses electrons) at an anodic site while the oxidising agent is reduced at a cathodic site. In the presence of an electrolyte, which is essentially any moisture containing dissolved ions, the reaction proceeds continuously.

Corrosion affects practically all structural metals used in industry: carbon steel, stainless steel, aluminium, copper, zinc and, in specific environments, even titanium. Each corrodes through mechanisms specific to its chemistry and its environment.

What is rust?

Rust is the common name for the reddish-brown iron oxide compounds that form when iron or steel reacts with oxygen in the presence of water. The main constituents are hydrated iron(III) oxide (Fe₂O₃·nH₂O) and iron(III) oxide-hydroxide (FeOOH). Rust is chemically distinct from the parent metal: it is porous, brittle and generally poorly protective. Unlike the dense, adherent passive films that protect stainless steel and aluminium, rust does not stop further corrosion. As it forms and flakes, fresh steel is exposed to further attack.

Key characteristics of rust:

• Only iron and iron-based alloys rust; other common structural metals form different corrosion products
• Rust occupies several times the volume of the iron it replaces, depending on the oxides formed, which is why corroding reinforcement cracks and spalls the surrounding concrete
• Rust is hygroscopic: it absorbs moisture, which accelerates further corrosion
• Rust in pits and crevices retains chloride and sulfate contamination, a major cause of coating failure when rusted steel is incompletely prepared before recoating

What types of corrosion affect steel structures?

Rust is the most visible form of corrosion on steel, but several distinct mechanisms affect industrial steel structures. Knowing which are present determines the protective strategy:

What are the ISO 8501-1 rust grades?

ISO 8501-1 defines four rust grades (A, B, C and D) that describe the initial condition of uncoated steel before surface preparation:

• Grade A: steel surface largely covered with adhering mill scale, with little if any rust. Mill scale is an oxide layer formed during hot rolling. It is cathodic to the underlying steel, so if it is left on and the coating is breached, the exposed steel corrodes preferentially at the break.
• Grade B: steel surface that has begun to rust and from which the mill scale has begun to flake.
• Grade C: steel surface on which the mill scale has rusted away or can be scraped off, with slight pitting visible under normal vision.
• Grade D: steel surface on which the mill scale has rusted away and general pitting is visible under normal vision. This is the most demanding starting condition.

Preparation grades (Sa grades for blast cleaning, St 2 and St 3 for hand and power tool cleaning) are assessed against these initial conditions. The required preparation grade depends on the coating system: many high-performance industrial systems specify Sa 2½ (SSPC-SP 10), while surface-tolerant maintenance coatings may accept less. The coating data sheet governs. Reaching a given grade on Grade D steel with deep pitting takes considerably more effort and time than on Grade B steel.

What does the rust grade tell you about surface preparation?

The rust grade gives practical information before preparing a surface for coating or recoating:

• Grade A steel: mill scale must be removed for most high-performance coating systems, because any break in the scale creates a galvanic cell. Abrasive blasting is the standard route. Hand and power tool cleaning to SSPC-SP 2 / SP 3 (St 2 / St 3) does not remove tightly adherent mill scale; impact power tools working to SSPC-SP 11, such as bristle blasting, can.
• Grade B–C steel: rusted but without severe pitting. Blast cleaning to Sa 2½ is standard; in maintenance work, the Bristle Blaster® gives cleanliness comparable to Sa 2½ / SSPC-SP 10 with a 65–85 µm Rz anchor profile on standard steel in documented tests, depending on steel grade, rust grade, belt and technique.
• Grade D steel: deep pitting is present. Use blast cleaning or a two-step mechanical approach (bulk corrosion removal, then final cleaning and profiling), pay particular attention to pit bottoms, and control soluble salts.

Key takeaways

• Rust is a specific form of corrosion: the iron oxide product of steel reacting with oxygen and water. Corrosion is the broader category of electrochemical metal degradation that affects all structural metals.
• Several corrosion types affect steel structures: uniform rusting, pitting, galvanic, crevice, erosion-corrosion, stress corrosion cracking and MIC. Each has different causes, locations and preparation implications.
• ISO 8501-1 rust grades A–D describe the initial condition of steel, from adhering mill scale (A) to general pitting (D). The rust grade determines the effort needed to reach a given cleanliness grade.
• Many high-performance industrial coating systems specify Sa 2½ (SSPC-SP 10); the coating data sheet sets the actual minimum.
• Pitted steel retains corrosion products and soluble salts that are hard to remove by any single method. Thorough preparation and soluble salt testing are essential before coating.

Related articles

• Corrosion Protection for Steel Structures: A Practical Guide
• Pitting Corrosion: What It Is and How to Address It
• Complete Guide to SSPC Surface Preparation Standards
• How to Keep Steel Corrosion-Free

Frequently asked questions

Is rust the same as corrosion?

No. Corrosion is the general term for a metal degrading through reaction with its environment, and it affects steel, aluminium, copper, zinc and other metals. Rust is the specific corrosion product of iron and steel: hydrated iron oxides and oxide-hydroxides. All rust is corrosion, but aluminium or copper corrosion is not rust.

Can stainless steel rust?

Yes, under the wrong conditions. Stainless steel relies on a thin passive chromium oxide film. Chlorides, crevices, contamination with carbon steel particles or damage to the film can cause pitting, crevice corrosion or surface rust staining. That is why stainless surfaces should be prepared with stainless steel tools, not carbon steel brushes or belts.

What are the ISO 8501-1 rust grades?

ISO 8501-1 describes four initial conditions of uncoated steel: A, adhering mill scale with little or no rust; B, beginning to rust with mill scale starting to flake; C, mill scale rusted away or scrapable, with slight pitting; and D, mill scale rusted away with general pitting visible. Preparation grades are then judged against these starting conditions.

Why is rusted, pitted steel harder to prepare for coating?

Pits trap corrosion products and soluble salts such as chlorides and sulfates. If they remain under a new coating they can cause osmotic blistering and renewed corrosion. Pitted steel therefore needs more thorough preparation, careful attention to pit bottoms and soluble salt testing before coating, whichever preparation method is used.