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Improving Safety and Environment in Cleaning and Passivation of

Stainless Steel

R. J. Kremer (V) and J. R. Eliasson (V)

Stainless steel passivation is a mysterious process to many, but one that is important in the marine industry to ensure

the full benefit from corrosion resistant steel alloys. It is a very important tool for the purpose of rejuvenating the

stainless tank surfaces, maximizing flexibility and loading opportunity. The use of stainless steel in tanker

construction is discussed. The principles and processes of passivation are explained. Traditional nitric acid based

passivation methods are compared to more recent citric acid based methods, including differences in aspects such as

safety, disposal issues, ease of use, and resulting corrosion protection. Focus is given to the challenges involved in

high corrosive exposure such as the marine environment, maintaining a good surface, and recovering from corrosion

when it has occurred. Passivation testing methods and industry specifications and standards for stainless steel

passivation are discussed.

KEY WORDS: Passivation; stainless steel; corrosion; citric

acid; tank maintenance.

INTRODUCTION

Stainless steel is a very important material to modern society. It

is used in many applications where high strength and corrosion

resistance are required, including the maritime cargo shipping

industry. Many people not familiar with the industry or

metallurgy are often surprised to hear that even stainless steel

can corrode. Since the main constituent of stainless steel is iron,

passivation is used to gain the maximum potential of its

corrosion resistance. This process was traditionally done with

nitric acid, but the more recently introduced citric acid process

shows many benefits and therefore has rapidly gained in

popularity. This paper will discuss the use of stainless steel in

shipbuilding, the history of citric acid passivation in industry,

the advantages of using citric instead of nitric acid, special

variations and procedures that are used when needed, and

methods for testing the effectiveness of a passivation treatment.

BASICS OF PASSIVATION AND STAINLESS

STEEL

Passivation by a chemist's definition is to make a material

resistant to chemical reactions, such as corrosion. Most metals

self-passivate upon exposure to air, forming a thin layer of

stable metal oxide on the surface. Iron is an obvious exception.

The purpose of the stainless steel alloy is to introduce this

benefit to steel, primarily through the addition of chromium,

also nickel in the austenitic 300 grades, and other metals

depending on the exact grade. The presence of these metals in

the alloy reduces the corrosion potential of the surface to some

extent, and the passivation treatment reduces it much further.

ASTM's (American Society for Testing and Materials) stainless

steel passivation specification A967 defines passivation as the

removal of iron and other exogenous materials from the surface,

which creates an iron-depleted layer of these other metals.

When there is no free iron exposed on the surface, rust cannot

form. The nonferrous metals are also able to form a higher

quality oxide layer that protects the underlying steel from

normal environmental conditions. The same principle holds true

for completely nonferrous alloys that have acquired iron

contamination on the surface during tooling and other

manufacturing processes.

Pickling is another related process, often conflated with

passivation, which uses acid to etch the surface layer of the steel

in order to remove unwanted discolorations or scale to produce a

uniform appearance. Pickling may be performed on both mild

steel and stainless steel, for stainless steel it requires harsh acids

that are able to attack the more chemical-resistant surface. It is

a much more aggressive process than passivation, though

passivation is often attained as well during a pickling process. If

the cosmetic appearance of the stainless steel is already

acceptable or deemed unimportant, the passivation process can

be performed in order to attain maximum corrosion resistance

without necessarily requiring a pickling step.

STAINLESS STEELS IN SHIP CONSTRUCTION The construction of chemical tankers involves the use of mild

carbon steels for the hull with integral stainless steel cargo

tanks. This invariably means using both steel materials, often

referred to as "black" and "white" steel respectively, in the same

working environment, as well as welded directly to each other.

This means that there is a large inherent risk that the stainless

steel will become contaminated by iron particles.

The shipyard receives both types of steel from steel mills. Then

the steel is cut, welded, and handled in the yard to make up

stainless steel cargo tanks and the mild steel hull, sometimes

using the same equipment for both. This transfers iron from the

mild steel onto the surface of the stainless steel. Small dust-

sized particles of the mild steel in the air produced by the cutting

and welding also deposit onto the nearby stainless. This iron

contamination on the surface of stainless steels reduces the

quality of the natural corrosion resistance. Given the

environment it is virtually impossible to keep the stainless steel

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surfaces free from iron contamination, and for that reason

passivation is a necessary part of the construction process.

Welds on stainless steel also have inherently reduced corrosion

resistance due to the heat affected zone, where the heat from

welding brings the metal not quite to the full melting point of

the alloy, but still hot enough to allow the alloy metals to

resegregate. A passivation treatment on the heat affected zone

restores the corrosion resistance.

Cargo lines and other pipes are often not accessible internally

after welding, and are welded using a shielding gas to reduce the

formation of thick oxides during the welding, but also are

normally re-circulated with pickling and passivation solutions to

remove any that does form and to ensure the corrosion

resistance is restored at the weld sites. The stainless steel cargo

tanks on board are used to carry a variety of cargoes including

various qualities of phosphoric acid, sulfuric acid, and organic

acids such as acetic acid and formic acid. The resistance of the

stainless steel must satisfy such aggressive service, looking for

the best possible resistance for lowest possible risk of damage,

and for that reason a passivation process is employed. Needless

to say, pickling and passivation, when done using very

dangerous acids, poses a significant risk in a shipyard

environment, a risk that is much reduced when using citric acid

solutions.

USE OF PASSIVATION TO ELIMINATE CARGO

HISTORY The most efficient use of a ship’s cargo space is to carry loads in

transit in all directions of travel, rather than just importing cargo

to a port, you want to export cargo from there as well. Stainless

steel tanks are extremely beneficial for this due to their

versatility. They can be used to carry various acids as

previously mentioned, but also oils such as vegetable oil and

coconut oil, and many other cargos as well. This provides the

highest degree of flexibility to the cargo ship.

The stainless tanks must of course be cleaned before accepting a

different type of cargo. The natural cleaning media onboard a

ship is seawater, as it is readily available, however seawater is

high in chloride content, which is detrimental to the stainless

surface. Repeated washing with hot seawater causes

degradation of the passive layer, and periodic passivation

treatments restores it.

Also, the last three cargos carried in a stainless tank are taken

into consideration regarding what cargos it can be safely used

for. This is referred to as the cargo history. For example, an

edible cargo may not be allowed if the tank was used three or

fewer shipments prior to haul a hazardous material, due to

concerns of lingering residue. This reduces the flexibility of

cargos for that tank. The solution was suggested, and approved

by most regulatory agencies (mainly FOSFA [Federation of

Oils, Seeds and Fats Associations] and NIOSH [National

Institute for Occupational Safety and Health]), that the

passivation process cleans and rejuvenates the surface of the

stainless steel and as such eradicates the past cargo history,

ensuring no hazardous residue remains. This is now the most

common reason for passivation of stainless steel cargo tanks.

HISTORY OF CITRIC ACID PASSIVATION The use of citric acid for passivation of stainless steel was first

discovered more than thirty years ago by the Adolf Coors

brewing company in Germany (Olsson, Parra, and Ragno,

1983). They had begun using stainless steel kegs for their beer,

but they discovered that the first time each keg was used the

beer gained a metallic taste. This was due to insufficiently

successful passivation of the kegs leaving iron on the metal

surface, which then was taken up into the beer. This effectively

passivated the kegs for future use, but at the expense of much

wasted beer. A study was run testing many chemicals for their

stainless steel passivation potential, in which citric acid emerged

as the clear winner, as shown in Table 1.

Table 1. Coors Passivation Test of Effects on Beer Flavor

Passivating Agents

(All at 70°C / 158°F)

S. S. Alloys Flavor Results

4% Citric Acid 304 & 304L Acceptable

2% Citric Acid 304 & 304L Unacceptable

4% Sulfamic Acid 304 & 304L Unacceptable

2% Sulfamic Acid 304 & 304L Unacceptable

4% Tannic Acid 304 & 304L Unacceptable

2% Tannic Acid 304 & 304L Unacceptable

4% Phosphoric Acid 304 & 304L Unacceptable

2% Phosphoric Acid 304 & 304L Unacceptable

Calcium Oxalate 304 & 304L Unacceptable

Ozonated Water 304 & 304L Unacceptable

This study was re-discovered several years later while

researching a technically and economically acceptable solution

for a company that was using nitric acid to passivate their

stainless steel springs. They were under pressure from OSHA

(Occupational Safety and Health Administration) to eliminate

the nitric acid from their plant due to safety concerns. Further

experimentation led to improved formulations and they went

into production with citric acid passivation baths. The use of

citric acid for passivation caught the interest of many other

companies, which then followed suit, eager to rid themselves of

nitric acid.

The prevalent standard reference for the passivation of stainless

steel at the time was the military specification QQ-P-35c, which

of course described only nitric acid processes. Many companies

eager to switch to citric acid were unable to because they were

beholden to this spec. As it happened, the Department of

Defense was already at the time working with industry groups to

phase out military specs in favor of industry versions. ASTM

developed the new specification A967, adding the newly

introduced citric acid methods alongside the established nitric

methods, and QQ-P-35c was withdrawn by the DoD

(Department of Defense) in 1998 in favor of the new standard.

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In addition, the older ASTM A380, a standard practices

document for cleaning stainless steel, as of 2013 allows a

crossover with A967 to provide for citric acid passivation as

well as nitric.

COMPARISON WITH NITRIC ACID

PASSIVATION Citric acid passivation offers many advantages over nitric acid

passivation. Perhaps foremost among them is safety, both in

handling during the passivation process and in storage on board

ship. Nitric acid is very hazardous while citric acid is quite

benign, in fact while skin contact with nitric acid causes

catastrophic chemical burns, skin contact is generally not a

problem with citric acid. Though of course, good chemical

handling practices dictate the use of protective gloves and

goggles anyway, just to be safe. Nitric acid also gives off

harmful fumes, from general fumes that cause corrosion in the

surrounding structure and equipment (often requiring costly

maintenance as a result) to large red clouds of very toxic fumes

if the wrong reaction is accidentally set off. Citric acid, on the

other hand, is a solid, so the only fumes that can be generated

from a citric acid passivation process is harmless water vapor.

Disposal of nitric acid is another difficult issue, as it and the

associated rinse water are designated hazardous when used to

passivate stainless steel, and will contain heavy metals (such as

hexavalent chromium) that are also considered hazardous,

leading to high waste disposal costs. Citric acid, on the other

hand, is not hazardous. As shown in Table 2, when used on

stainless steel it only removes the iron, not the nickel,

chromium, or other metals present. This not only prevents it

from becoming environmentally hazardous waste, but is an

additional benefit for the stainless steel as it means there is no

worry to the organization performing the passivation that there

could be any accidental etching of the surface if the steel is in

contact longer than the prescribed time.

Table 2. Metal Content Of Citric Acid Bath After 30 Days

Passivation Of 316L SS

Citric

Acid

Titra-

tion

Metals in Citric Acid Solution (mg/L)

Fe Ni Cr Cd Mn

Before

Passiv-

ation

4.44% <0.05 <0.05 <0.05 <0.02 <0.05

After

Passiv-

ation

4.40% 0.72 <0.05 <0.05 <0.02 <0.05

With only the iron being removed, citric acid passivated

stainless has a thicker layer of chromium on the surface, thus

allowing a better chrome oxide layer to form, as shown in

Tables 3 and 4 via Electron Spectroscopy for Chemical Analysis

and Auger Electron Spectroscopy data. Tests run at University

College Cork in Ireland using several x-ray spectroscopy

techniques arrived at similar conclusions (O'Laoire, Timmins,

Kremer, Holmes, and Morris, 2006). All of this makes the citric

acid solutions the safe, technically superior, and more

economical solution.

Table 3. ESCA Evaluation of Passivation Process

Citric

Acid

Sample 1

Citric

Acid

Sample 2

Nitric

Acid

Sample 1

Nitric

Acid

Sample 2

Chrome

oxide / Iron

oxide ratio

5.5 5.3 2.1 2.0

Chrome /

Iron ratio

2.5 2.5 1.4 1.4

Table 4. AES Depth Profile Results

Oxide

Thickness

Max. Depth of

Enrichment

Depth of

Enrichment

Citric Acid

Sample 1

27.0 Å 18.0 Å 17.0 Å

Citric Acid

Sample 2

28.0 Å 19.0 Å 17.0 Å

Nitric Acid

Sample 1

21.0 Å 13.0 Å 12.0 Å

Nitric Acid

Sample 2

17.0 Å 11.0 Å 11.0 Å

For the aforementioned reasons citric acid is much easier to use

for passivation. It can also be used with higher heat than is safe

with nitric acid, allowing for a faster process, and the useful life

of each batch is longer than with nitric acid, resulting in a lower

volume of chemicals needed to perform the same passivation

job. Additionally, the concentration of citric acid required for

passivation is lower than that of nitric acid (four to ten weight

percent versus at least twenty volume percent), again reducing

the volume of chemicals needed.

VARIATIONS OF THE CITRIC ACID

PASSIVATION FORMULA Thickening agents have long been used with nitric acid and

nitric/hydrofluoric acid mixes to form a paste that can be used to

treat localized stainless surfaces and areas where recirculating

spray methods do not apply, as opposed to the process used for

the large storage tanks. The same can be done with citric acid,

and the same safety and disposal benefits apply. This often

comes into play with welds, as the heat affected zone on a

stainless weld can be susceptible to corrosion if not passivated

afterwards. Smaller stainless parts can be passivated by

immersing them in a small bath of the acid.

PROBLEMATIC FOR PASSIVATION A rising problem in the stainless steel industry is low quality

stainless steel entering the market, often due to increased

amounts of scrap iron used in production. To meet the

specification for any particular grade of stainless steel, an alloy

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must meet the prescribed percentages for chromium and other

additives, and most people just presume that the remainder is

iron. However this is not necessarily true, and inclusions of

other materials greatly affect the corrosion resistance. This can

be seen in stainless fixtures used in harsh environments such as

swimming pools and marine environments. Traditionally some

versions of 304 were sufficient, but today such grades are often

inadequate and the more expensive higher grade 316 must be

used to achieve sufficient corrosion protection. The

concentration and frequency of addition of chlorine, which

negatively affects stainless steel, has also increased over time in

stainless steel contact areas. Selecting high quality stainless

steel, as opposed to borderline grades or steel that was more

poorly manufactured, avoids a lot of problems.

PASSIVATION TESTING Evaluating the corrosion protection imparted by passivation is a

key item in finding and maintaining a good process. Several

passivation tests are available. Most tests are designed for small

parts or test coupons, but some are useful for large or in-service

items. The most convenient test for speed and ease of use is the

copper sulfate test. For this process, a few drops of copper

sulfate solution are applied to the stainless steel surface. If free

iron is present, copper will deposit on the surface and a color

change will be visible after several minutes. The potassium

ferricyanide test works in a similar manner. Both tests,

however, can give false negative results (test failures) on some

grades of stainless, notably 400 series stainless steels, due to the

lower chromium content in those alloys.

The "damp cloth" test involves exposing the stainless surface to

a cloth soaked in distilled water for an hour, after which the

surface is examined for evidence of corrosion. This is similar to

distilled water immersion tests used for smaller stainless parts.

Test methods such as the high humidity test and salt spray test

also seek to accelerate corrosion, and typically require sending

the parts or test coupons out to a laboratory that has the

necessary apparatus. Test results of a more quantitative manner

can be acquired via electron spectroscopy for chemical analysis

and auger electron spectroscopy, which can report the chemical

composition of the surface layer, indicating the level of

chromium enrichment (due to iron depletion from the

passivation treatment) and the amount of chrome oxide

formation.

CONCLUSIONS

The advantages of using citric acid for passivation of stainless

steel combined with the knowledge of how to handle special

application processes produces the best possible corrosion

resistance for the maritime shipping industry. Using nitric acid

is no longer necessary for passivation, and those in the industry

who switch to citric acid based solutions can benefit greatly in

improved surface quality, safety, and cost savings. This is borne

out by tests run by thousands of companies worldwide.

REFERENCES

H. OLSSON, J. PARRA, and J. RAGNO “Stainless

Steel Flavor Contribution to Beer.” MBAA Technical

Quarterly, Vol. 20, No. 3, 1983, p 102-105.

C. O'LAOIRE, B. TIMMINS, L. KREMER, J. D.

HOLMES, and M. A. MORRIS “Analysis of the

Acid Passivation of Stainless Steel.” Analytical

Letters, 39:11, 2255 – 2271 (2006)