ALLOY 28 CORROSION RESISTANCE

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Alloy 28 Corrosion Resistance (UNS N08028)

Alloy 28 is a high-alloy austenitic stainless steel designed for service in highly corrosive environments. Its advanced metallurgy provides exceptional resistance against general corrosion, pitting, crevice corrosion, stress corrosion cracking (SCC), and intergranular corrosion.

Chemical Composition of Alloy 28

Material	C (max %)	Cr (%)	Ni (%)	Mo (%)	Cu (%)	W (%)	Co (%)	Others
Alloy 28	0.02	27	31	3.5	1.0	-	-	-
Alloy 904L	0.02	20	25	4.5	1.5	-	-	-
UNS N08020	0.07	20	34	2.5	3.3	-	-	Nb
Alloy 825	0.05	21.5	42	3	2.3	-	-	Ti
Alloy G	0.03	22	45	6.5	2	1.0	2.5	Nb
Alloy C	0.08	15.5	54	16	-	4	2.5	-

1. General Corrosion Resistance

Alloy 28 was originally developed for phosphoric acid production, particularly in heat exchangers, where corrosive conditions are severe. Laboratory tests confirm Alloy 28’s superior resistance to chlorides (Cl⁻) and fluorides (F⁻) compared to other high-alloy materials.

Corrosion Rates in Super Phosphoric Acid (200°C / 390°F, 20 days)

Alloy	Corrosion Rate (mm/year)	Corrosion Rate (mpy)
Alloy 28	0.03	1.2
Alloy 904L	0.10	4.0
UNS N08020	0.23	9.2
Alloy G	0.03	1.2

Key Points:

• Alloy 28 exhibits superior corrosion resistance in contaminated acids.
• Its resistance remains strong even at elevated temperatures.

Corrosion Resistance in Other Acids

• Sulphuric Acid: Highly resistant, including chloride-contaminated solutions; superior to 904L and SAF 2507 at >20% concentration.
• Hydrochloric Acid: Performs better than lower chromium/molybdenum stainless steels.
• Hydrofluoric & Hydrofluosilicic Acid: Excellent resistance, suitable for impure environments.
• Acetic & Formic Acid: Resistant at all temperatures; outperforms AISI 316/317L in contaminated solutions.
• Nitric Acid: ASTM A262 Practice C tests yield corrosion rates <0.15 mm/year (6 mpy).
• Sodium Hydroxide: Resistant to erosion corrosion; suitable alternative to pure nickel at moderate temperatures.

General Corrosion and SCC in Sodium Hydroxide

Grade	SCC (Boiling 43% NaOH + 6.7% NaCl, 142°C / 288°F, 500h)	General Corrosion Rate (mm/year) 99°C / 210°F	135°C / 275°F
Alloy 28	No	0.008 (28% NaOH)	0.074 (43% NaOH)
Alloy 800	Yes, cracks <120 µm	0.011	0.397
Alloy 904L	Yes, cracks <150 µm	0.013	0.301

2. Pitting Corrosion

Alloy 28 demonstrates high critical pitting temperature (CPT) in chloride-bearing environments, outperforming Alloy 904L and Alloy 825 even in acidic chloride solutions.

3. Crevice Corrosion

ASTM G-48 Method B (6% Fe³⁺ chloride) testing shows Alloy 28 has better crevice corrosion resistance than Alloy 825.

4. Stress Corrosion Cracking (SCC)

Ordinary austenitic steels (AISI 304/316) are susceptible to SCC in chloride-bearing solutions >60°C (140°F). Alloy 28, with 27% Cr and 31% Ni, shows excellent SCC resistance in laboratory and field conditions, including environments containing hydrogen sulfide.

5. Intergranular Corrosion

TTC diagrams (ASTM G-28) confirm Alloy 28 can remain in the critical 600–700°C (1100–1300°F) range for at least 30 minutes without intergranular attack. Less susceptible to welding-related corrosion than Alloy 904L.

6. Key Advantages of Alloy 28

• Very high corrosion resistance in strong acids
• Excellent SCC and intergranular corrosion resistance
• High resistance to pitting and crevice corrosion
• Good weldability
• Suitable for highly aggressive chemical processes

Standards & Approvals

UNS Number	N08028
ISO Number	4563-080-28-I
EN Number / Name	1.4563 / X1NiCrMoCu 31-27-4
W.Nr.	1.4563
Product Standards	ASTM B668, EN 10216-5, ASTM B709, EN 10088-2, ASTM A403
Approvals	ASME Section I, III, VIII, NACE MR0103, NACE MR0175/ISO 15156

7. Applications of Alloy 28

Alloy 28 is widely used in:

• Phosphoric and sulfuric acid production
• Hydrochloric, hydrofluoric, and nitric acid processes
• Chemical processing equipment in high-chloride environments
• Oil & gas applications requiring SCC-resistant materials

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