What is Salt Spray Test Machine
Most of the corrosionof metallic material soccursin the atmosphere,which contain scorrosive components and corrosive factors such as oxygen, humidity, temperature changes and pollutants. The corrosion of the metal salt by the salt spray is mainly caused by the electrochemical reactionof the conductive salt solution into the metal, forming a micro-battery system of “low-potential metal-electrolyte solution-high-potential impurity”, electron transfer occurs, and the metal as the anode is dissolved. A new compound, the corrosive, is formed. The main function of the salt spray corrosionprocess is chloride ion.It has a strong penetrating power and easily penetrates the metal oxide layer into the metal to destroy the passive state of the tal.Atthesametime,thechlorideionhasasmallhydrationenergy.Itiseasilyadsorbedon the metalsurfaceandreplacestheoxygenin theoxidelayer oftheprotective metaltodestroy the metal. Read about PP Spray Cabin Machine
The salt spray test is an accelerated corrosion resistance evaluation method for an artificial atmosphere. It atomizes a certain concentration of salt water; it is sprayed in a closed incubator to reflect the corrosion resistance of the sample to be tested by observing the changeofthesampletobeplacedintheboxforaperiodoftime.Itisanacceleratedtest.The method has the salt concentration of the chloride in the salt spray environment, which is several times or several times of the salt content of the general natural environment, so that thecorrosionrateisgreatlyimproved,andthesaltspraytestisperformedontheproduct,and the time for obtaining the result is also greatly shortened.
When testing a product sample in a natural environment, the corrosion time may take one year or even several years, and in a simulated simulated salt spray environment, similar results can be obtained in a few days or even hours.
The salt spray test is mainly divided into four types: Neutral salt spray test (NSS); Acetate spray test (AASS); Copper accelerated acetate spray test (CASS); Alternating salt spray test.
Evaluation of Salt Spray Test Method
The results of the Salt Spray Test Method include: 1.Rating method; 2.Corrosion appearance evaluation method; 3.Weighing method.
The percentage of the ratio of corrosion to the total product is divided into several levels according to a certain method, and a certain level is used as the basis for the qualification. It is suitable for the evaluation of the flat sample. Such methods are used to evaluate Salt Spray Test Method results, such as GB/T6461-2002, ISO 10289-2001, ASTM B 537-70 (2013), and ASTM D 1654-2005.
Over lay classification and subjective evaluation
Classification of cover damage types A Spot or color change caused by damage to the cover B Hard to see the darkness caused by corrosion of the overlay C Corrosion products of anodic coating D Corrosion products of cathode coating E Surface pitting F Broken, peeled, peeled off G Bubbling H Cracking I Crack J Chicken claw or star flaw
Subjective evaluation of the degree of damage of the cover vs Very mild s Mild m Moderate x Severe The protection rating is expressed as:RA/— For example, if the slight rust exceeds 1% of the surface and is less than 2.5% of the surface, it is expressed as:5/—
The appearance rating is expressed as:—/RA value+Subjective evaluation + cover damage level Such as: moderately spotted, the area is more than 20%, expressed as: —/2mA
The performance rating is expressed as:RA value+Subjective evaluation + cover damage level For example, the sample does not show corrosion of the base metal, but there is mild corrosion of the anodic cover layer less than 1% of the total area, expressed as 10/6sC
Photograph of the capping layer of the base metal (Source: ISO 10289-1999)
Corrosion Appearance Evaluation Method
It is a qualitative method for judging whether the product is corroded after the salt spray corrosion test. For example, JB4159-1999, GJB4.11-1983, and GB/T4288-2003 use this method to evaluate the salt spray test results.
Corrosion characteristics of common plating parts after Salt Spray Test Method
Plating type Corrosive feature Corrosive picture Zinc plating on steel parts Gray or black plating corrosives and brown rust Chromium plating on steel parts Gray or black plating corrosives and brown rust Cadmium plating on steel parts Brown steel rust Copper plated silver Green copper rust
Copper plated tin Gray plating corrosive green copper rust
Test Temperature and Humidity
The critical relative humidity of metal corrosion is approximately 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquesce to form an electrolyte with good electrical conductivity. When the relative humidity is lowered, the concentration of the salt solution will increase until the crystalline salt is precipitated, and the corrosion rate is correspondingly lowered. When the temperature rises, the molecular motion is intensified, and the high salt spray corrosion rate is faster. The International Electrotechnical Commission pointed out that for every 10 °C increase in temperature, the corrosion rate is increased by 2 to 3 times, and the conductivity of the electrolyte is increased by 10 to 20%. For the neutral salt spray test, it is generally considered appropriate to select the test temperature at 35°C.
When the concentration is below 5%, the corrosion rate of steel, nickel and brass increases withtheincreaseofconcentration;whentheconcentrationismorethan5%,thecorrosionrate of these metals decreases with the increase of concentration. This is because, in the low concentration range, the oxygen content increases as the salt concentration increases; when the salt concentration increases to 5%, the oxygen content reaches a relative saturation, and if the salt concentration continues to increase, the oxygen content decreases accordingly. As theoxygencontentdecreases,thedepolarizationabilityofoxygenalsodecreases,thatis, the corrosion effect is weakened. For metals such as zinc, cadmium, and copper, the corrosion rate always increases with the concentration of the salt solution
The sedimentation direction of the salt spray is close to the vertical direction. When the sample is placedhorizontally, its projected area is the largest, and thesample surface receives the mostsalt spray, sothecorrosionis themost serious.Theresults showthatwhen the steel plate is at an angle of 45 degrees with the horizontal line, the weight loss per square meter is 250 g. When the planeof the steel plate is parallel to the vertical line, the weight loss of corrosion is 140 g per square meter. The GB/T 2423.17-93 standard stipulates that the flat sample should be placed at an angle of 30 degrees from thevertical.
The lower thepH, the higher the hydrogen ion concentration in the solution, andthe stronger theacidity,thestrongerthecorrosion.Theneutralsaltspraytest(NSS)hasapHof6.5to7.2. The pH of thesalt solutionchanges due to environmental factors. To this end, the salt spray test standard at home and abroad has specified the pH range of the salt solution, and the method of stabilizing the pH value of the salt solutionduring the test is proposed to improve the reproducibility of the salt spray test results.
Salt Spray Settlement and Spray Method
The finer the salt spray particles, the larger the surface area formed, and the more oxygen is adsorbed, the stronger thecorrosivity. Conventional spray methods include a gas jet method and a spray tower method. The most obvious disadvantage is that Salt Spray Test Method has a poor uniformity and thesalt spray particles have a large diameter. Different spray methods also have aneffect on the pH of the salt solution.
The salt spray test is divided into two categories, one is the natural environmental exposure test, and the other is the artificial accelerated simulated salt spray environment test. The artificial simulated salt spray environment test uses a test equipment with a certain volume space, the Salt Spray Testing Chamber, to artificially use the salt spray environment in the volume space to assess the salt spray corrosion resistance quality of the product. . Compared with the natural environment, the salt concentration of the chloride in the salt spray environment can be several times or several times that of the salt spray content of the general natural environment, so that the corrosion rate is greatly improved, and the product is subjected to a Salt Spray Testing Chamber to obtain a result. The time is also greatly shortened. If a product sample is tested in a natural exposure environment, it may take 1 year to corrode, and in a simulated simulated salt spray environment, as long as 24 hours, similar results can be obtained.
The artificial simulated salt spray test includes a neutral Salt Spray Testing Chamber, an acetate salt spray test, a copper salt accelerated acetatespray test, and an alternating salt spray test.
(1) Neutral salt spray test (NSS test) is one of the earliest applications in the field of acceleratedcorrosiontest.It usesa 5%aqueous solutionof sodium chloride solution, and the pH of thesolution is adjusted to a neutral range (6-7) as a solution for spraying. The test temperature is taken at 35 ° C, and the sedimentation rate of the salt spray is required to be between 1 and 2 ml / 80 cm² · h.
(2)Theacetatespraytest(ASStest)wasdevelopedonthebasisofaneutralsaltspraytest.It is to addsome glacial acetic acid tothe 5% sodium chloride solution, the pH value of the solutionis reduced to about 3, the solution becomes acidic, andthe salt mist formed finally changes from neutral salt spray to acidity. Its corrosion rate is about 3 times faster than the NSS test.
(3) Copper salt accelerated acetate spray test (CASS test) is a rapid salt spray corrosion test developed recently in foreign countries. The test temperature is 50 ° C. A small amount of copper Salt Spray Test Method – copper chloride is added to the salt solution to strongly inducecorrosion. . Its corrosion rate is about 8 times that of the NSS test.
Conclusion – Salt Spray Testing Chamber
The specifictime isconverted to:
Neutralsaltspraytest24h ⇌ natural environment 1year Acetate spraytest24h ⇌ naturalenvironment3 years Coppersaltaccelerated acetatespraytest24h ⇌ naturalenvironment8 years
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